Tag Archives: exposure

ISO Confusion Once Again!

I’m going to keep bringing this up until people start to take note and understand that with an electronic camera ISO is NOT sensitivity.

With an electronic camera ISO is a guide to the required shutter speed and aperture needed to get the correct exposure. This is different to sensitivity. The ISO rating of a video camera and it’s sensitivity are closely related, but they are not quite the same thing. Because different gamma curves require different exposures the ISO rating for each gamma curve will be different even though the gain and actual sensitivity of the camera may be exactly the same.

Lets take the  Sony PXW-FS5 as an example.

If you shoot using the standard camera settings you should expose white at 90%, middle grey will be around 42% and skin tones typically around 70%. At 0dB gain the camera the camera will display an ISO equivalent rating of 1000 ISO. So let’s say you are using a light meter. You set it to 1000 ISO and it tells you you need an aperture of f5.6 to get the right exposure.

Now you change to S-Log2. If you do nothing else your white card will now be at around 75% and middle grey will be around 40%. At 0dB gain the camera will show an equivalent ISO of 3200 ISO.

But hang on – The camera is still at 0dB gain, so there is no change in sensitivity. .But the camera is over exposed, S-Log2 is supposed to be exposed with white at 59% and middle grey at 32%.

So we go to our light meter and change the ISO on the light meter from 1000 ISO to 3200 ISO. Because the light meter now “thinks” the camera is more sensitive by almost 2 stops it will tell us to close the aperture by nearly 2 stops. So we go to the camera and stop down to f10 and bingo, the image is exposed correctly.

But here’s the important thing – The camera hasn’t become any more sensitive. We haven’t replaced the sensor with a different, more sensitive one (as you would do with a film camera where you actually change the film stock). We are still at 0dB gain (even though the camera tells us this is the equivalent to a higher ISO).

The only reason that ISO number changes is so that if we were using an external light meter we would get the recommended exposure levels for the gamma curve we are using. In this example closing the aperture increase the highlight range that the camera would be able to cope with and this helps us get that full 14 stop range from the camera, although closing the aperture means less light on the sensor so the pictures end up a little noisier as a result – That is unless you choose to rate the camera at a different ISO by over exposing the log a bit.

ISO is useful, but you need to understand that it isn’t really sensitivity. After all we can’t change the sensors on our video cameras and that would be the only way to truly change the sensitivity. Any “sensitivity” change is really nothing more than a gain or amplification change. Useful but not the same as changing the actual sensitivity. Gain will make a dark picture brighter but it won’t allow you to see something that the sensor can’t detect.

It is much easier to understand dB gain with an electronic camera as it actually tells you exactly what the camera is doing and it is actually my recommendation that people use gain rather than ISO for all of the above reasons.  The use of ISO on electronic cameras is very badly understood, in part because it’s a largely meaningless term because it doesn’t tell us how sensitive the sensor is, how much gain we are using or how much noise we are adding. Give any experienced camera operator a camera and ask them how noisy will it be a 18dB gain and they will have a pretty good idea of what the pictures will look like. Give them the same camera and ask them how noisy will it be at 8000 ISO and they won’t have a clue.

The problem is ISO is trendy and fashionable as that’s what “cinematographers” use. But lets be honest with ourselves – we are using electronic video cameras, whether that’s a Red, Alexa or FS5 so really we should be using the correct terminology for an electronic camera which is gain. It would eliminate an aweful lot of confusion and tell us how much noise and grain our pictures will have. It’s noise and grain will levels will determine how good a clip looks and how much we can grade it, so we need to clearly understand how much gain is being added in camera and dB gian tells us this. ISO does not.

Side Note: Modern film stocks will often have 2 ratings, the ISO or actual measured sensitivity of the film stock plus the EI or Exposure Index which is the recommended setting for the light meter to get the best exposure. In some respects the ISO rating of a video camera is closer to the EI rating of a film stock. Perhaps we should stop calling it ISO and use the term EI instead, this would be me appropriate and signify that it is a reference for best exposure rather than true sensitivity.

UPDATE: A comment on facebook was why not display both ISO and Gain side by side. This is an obvious solution really. Why do camera manufacturers force us to choose either ISO or gain? Why can’t we use a hybrid of the 2? I see no technical reason why cameras can’t show both the gain and ISO at the same time – Problem solved.

Auto Knee when shooting with Rec-709.

Like many cameras the Sony PXW-FS7, PMW-F5 and F55 use an automatic knee circuit to help the camera handle strong highlights or overexposure when shooting using standard gamma curves such as Rec-709 (STD gamma 5). On some ENG cameras there is a very similar function  called DCC (Dynamic Contrast Compensation) which is often selected via the Camera/Bars switch.

On the FS7, F5/F55 and many others the Auto Knee is on by default out of the factory. It can be turned on and off in the cameras paint settings. In most normal shooting situations, if you are correctly exposed the auto knee does a good job of bringing bright highlights down out of clipping.  The auto knee threshold is at around 90% brightness. Expose with objects brighter than 90% in your scene and the auto knee starts to kick in.

The correct exposure for white, such as a 90% reflectivity white card or white piece of paper in Rec-709 is 90%. Skin tones, plants, walls, roads and in fact most objects will normally be below white or below 90%. However direct light sources, such as the sky or direct reflections such as shiny car body work will be brighter than white. So the knee should only ever effect objects brighter than white if you are exposed correctly.  So for most situations it should not effect skin tones and the majority of the scene, just the bright highlights.

The auto knee detects highlight levels above 90% and tries to keep the highlight range below clipping by adding contrast compression to the highlights. The amount of compression depends on how strong the highlights are. As a result the auto knee effect will vary with exposure. If you have a scene with only a few highlights there will be some knee compression and it’s effect will only be seen above approx 90%. If you then open the aperture or have a lot of highlights the auto knee will increase the highlight compression to compensate. If the highlight range becomes very large then the knee will not only increase the amount of compression but also lower the knee point so more and more of the upper exposure range is effected by the knee. In extreme cases the knee point may get as low as 70-80% and this then starts to effect skin tones.

To prevent rapid fluctuations of the contrast in the highlight range the auto knee has a slight delay. This can result in a vicious circle where you open the iris a bit to help brighten the shot. The shot gets brighter. Then a couple of seconds later you look at the shot again and because the knee has now adjusted the highlights after it’s delay period it looks different to how it looked at the moment you made the initial adjustment. So you adjust again…. then the knee adjusts again and so on. Sometimes this lag can make it tricky to get your highlights to look exactly how you want.

Another common auto knee effect is to see the brighter parts of an entire image change as a result of a change in only a small part of the scene. A typical example would be an interview with a window in the background. As the highlight level in the bright window changes, perhaps as the sun comes and goes from behind passing clouds, the knee tries to compensate and all of the highlights in the scene go up and down in brightness whether they are over exposed or not. This looks very strange and can ruin an otherwise good looking shot.

If you are shooting in a studio against a white background the auto knee makes it impossible to get a brilliantly bright uniformly clipped white background. You increase your exposure to make the white background extra bright and because that white is now above 90% the auto knee treats it as a highlight and tries to control it’s brightness. The more you open the aperture the more the knee pulls down the white background, it never reaches clipping. Eventually you get to the point where the knee starts to effect the skin tones but your white backdrop still isn’t clipped. The image doesn’t look great.

In these cases the best thing to do is to turn off the Auto Knee. If you go into the paint settings you will find the knee settings. In most cases leave the knee on (except perhaps for the white studio example), but turn OFF the auto knee function. The fixed level knee will still give you a good highlight range but eliminate the pumping or other variable knee effects. Note that the knee options have no effect if using a Hypergamma or log. They only come into paly with standard gamma.

Shooting in cold weather and shooting snow scenes. Updated.

A couple of years ago I wrote a guide to help people that might have to shoot in the cold.  I’ve recently updated this article and as I know many of you won’t have seen it before I’ve provided a link to the page below

LINK: This article deals with shooting in the cold and how that might effect your camera.

LINK: Some ideas and suggestions for clothing in very cold conditions.

Here also are some tips for shooting snow scenes with conventional gammas. Of course you can also shoot with log or raw, if you do just make sure your exposure is nice and bright for the best results, generally when there is a lot of snow around dynamic range isn’t a huge problem as the snow acts as a reflector to fill in a lot of shadows.

With conventional gammas such as Rec-709 exposing for snow is tricky. You want it to look bright, but you don’t want to overexpose and it’s very easy to end up with a lot of the bright snow in your scene up in the knee or highlights where it will be compressed and loose contrast. This makes the snow look odd as it will have no texture, it can all too easily look over exposed when in fact it is not. In reality, although we often think of snow as bright and white, often you really don’t want to expose it too high.  With Rec-709 if your camera has a high level zebra set them to 90% (Zebra 2 on most Sony cameras). This way you will get a zebra pattern on the snow as it starts to enter the compressed knee or highlight area. If you are using Sony’s cinegammas or hypergammas I would lower the highlight zebras to 80% -85%.

On overcast or flat light snow days I prefer not to use Hypergammas/Cinegammas  as the highlight roll off can make the snow look very flat unless you grade the images a little and boost the contrast in post. However on bright higher contrast snow days with clear skies and strong shadows the Hyoegammas/Cinegammas work very well. You may want to consider using a little bit of negative black gamma to put a bit more contrast into the image.

You also want your snow to look white, so do a manual white balance using a proper white card or better still a grey card. Don’t try to white balance off the snow itself as snow can reflect a lot of blue light and skew the white balance a bit.  If you are shooting during golden hour at the beginning or end of the day and want to retain that warm look you might want to use a 5600K preset rather than a manual white balance to capture either the golden hour light or the blue light that follows it.

If the overall scene is very bright you may need to watch your aperture. In most cases you don’t want to have the camera stopped down to an aperture of f11 or smaller.  Due to an effect called diffraction limiting, in HD, at f11 a 2/3″ camera will start to show a slightly soft image.  A 1/2″ sensor camera will be just starting to get slightly soft at f8.  In 4K/UHD a super 35mm camera will start to show a slightly softer image from f11 – f16. So use you ND filters to control you light levels so you do not have too small an aperture. You may need to add additional ND in very bright scenes to avoid diffraction limiting.

One last tip. If you are standing around in the cold and get cold feet you should find something to stand on. Small twigs and branches, a rubber car mat anything like that will help insulate your feet from the cold ground helping keep them warm.

Revealing Signal to Noise and Exposure experiment for PMW-F5, F55 and FS7.

Here’s a little experiment for you to try if you have a PMW-F5, PMW-F55 or PMW-FS7. It should help you understand a few key things about the way these cameras behave, notably:

1: Why ISO does not actually reflect the sensitivity of the camera.

2: Why it is beneficial to expose S-Log2 or S-Log3 brighter than the Sony recommended levels.

3: How to get the best possible S-Log footage.

4: Why S-log may be a poor choice for low light.

Ideally you will want to use an external waveform monitor connected to the cameras SDI output, but it is possible to use the built in waveform display.

Start with the camera in Custom mode. Choose “STD” gamma and Rec-709. Set the gain/ISO settings so that the camera is showing ISO.

Set the ISO to the base ISO (800 ISO on F5/FS7, 500 ISO on F55).

Expose a 90% white card so that white is 90% on the waveform display. This doesn’t need to be 100% accurate, you can use a piece of paper if you don’t have a proper white card. Don’t change the ISO/Gain, light the white card if you need to. Make a note of the aperture.

Now change the gamma selection to S-Log2, do not change the exposure.

Note how white now drops down to about 70% and also note that the ISO becomes 2000 ISO on an F5 or FS7 and 1250 ISO on an F55.

Think about this for a moment: If the ISO has gone up, how can white and the bulk of my image become darker?

Now switch the camera to show dB gain instead of ISO, the gain should be showing 0dB. Repeat the above switching from Standard 709 gamma to S-Log2 and note that the gain remains at 0dB for both rec-709 and S-Log2.

Think about this: The gain is the same for both 709 and S-log2 but the S-Log2 image is darker. As the gain is NOT changing then the sensitivity is not actually changing, so why does the ISO change?

If you were to use a light meter and start off with the light meter set to 800 (500) ISO the light meter would tell you to set the aperture to whatever it is you currently have to give the correct exposure in rec-709 with white at 90%.  If you had a light meter and you change the ISO setting on the light meter from 800(500)ISO to 2000(1250) ISO the light meter will tell you to close the aperture by 1.3 stops.

So, on your camera, while it is set to S-Log2 close the aperture from it’s original setting by 1.3 stops. Now you will find that white will be at the recording levels given by Sony for S-Log2 which is 59% for white and 32% for middle grey.

So what have we learnt from this? The gain is the same for both standard gamma and S-Log2, even though the ISO changes from 800(500) to 2000(1250) ISO. So the sensitivity and amount of noise coming from the sensor is the same in both cases. But the indicated ISO changes so that if you are using an external light meter, when you switch to S-Log the higher indicated ISO  will make the light meter tell you to close the aperture. This means there is less light falling on the sensor. This means that the recorded image will have a worse signal to noise ratio (noise remains the same, but signal is smaller).

To solution of course to this poorer signal to noise ratio is simply to open the aperture back up again by 1.3 stops. When shooting S-Log2 or S-Log3 using the CineEI mode I always recommend using 800EI on an F5 or FS7 or 640EI on an F55. This means your aperture becomes the same as it would be when shooting in vanilla Rec-709, the end result is the same, improved, signal to noise ratio. If you are not using CineEI or LUT’s, then expose white at 70%.

Understanding the all important “Signal to Noise Ratio”.

The Signal to Noise ratio is one of the key factors in determining the quality of a video or stills image. A noisy, grainy picture rarely looks as good as a low noise “clean” image. In addition it’s noise in your images will limit how far you can grade them before the picture quality becomes unacceptably poor.

Almost always what you want is the biggest possible signal with the least possible noise. In a video or film camera the signal is the desired image information or in simple terms the picture. While the noise is…. well….. the noise.

Once upon a time, when film cameras were normal for both still photography and film the noise in the pictures came primarily from the grain structure of the film stock. One of the great features of film cameras is that you can actually change that film stock to suit the type of scene that you are shooting. For low light you could use a more sensitive film stock that was actually truly more sensitive to light. However, often a very sensitive film stock will show more noise as the grain of more sensitive film is normally larger.

With video and digital stills cameras however things are quite different. You can’t normally change the sensor in a video camera and it’s the sensor that determines the sensitivity of the camera and it is the sensor that is the source of the majority of the noise.

Modern CMOS video sensors consist of two parts. The light gathering part and the readout part. The size of the pixels on a sensor is one of the key factors in determining the sensitivity and dynamic range. Small pixels are not good at capturing, converting and storing large numbers of photons of light or electrons of electricity.  Bigger pixels are much better at this, so big pixels typically mean better sensitivity and a better dynamic range. Each pixel is unique and as a result every pixel on the sensor will perform slightly differently. The signal stored in the pixels is a tiny analog signal that is easily disturbed by stray electric currents and variations in temperature. As a result of the small variations from pixel to pixel, the stray signals and heat, there is a small variation from moment to moment in the signal that comes off the pixel when it is read out and these variations are what we see as noise.

The analog signal from the pixels is passed to a circuit that converts it to a digital signal. The analog to digital conversion process normally includes some form of noise reduction circuitry to help minimise the noise. By carefully mapping the A to D circuity to the signal range the pixels provide, a sensor manufacturer can find the best combination of noise, dynamic and sensitivity. Once the signal has been converted to a digital one, the noise level, sensitivity and dynamic range is more or less locked in and can’t be changed (Some cameras have the ability to use slightly different A to D conversion ranges to help give improved noise levels at different brightness/dynamic ranges).

The bottom line of all this is that with the vast majority of video cameras the noise level is more or less fixed,  as is the sensitivity as we can’t actually swap out the sensor.

But wait! I hear you say…. My camera allows me to change the ISO or gain. Well yes it probably does and in both cases, ISO or gain, with a digital video or stills camera what you are changing is the cameras internal signal amplification. You are NOT making the camera more sensitive, you are simply turning up the volume. As anyone with any type of sound system will know, when you turn up the gain you get more hiss. This is because gain makes not only the desired signal bigger but also the noise. As a result adding gain or increasing the ISO is rarely a great thing to do.

So normally we want to use a digital camera at it’s native sensitivity wherever possible. The native sensitivity is where no gain is being added by the camera or 0dB. In ISO, well you need to find out what the native ISO is and be aware that different gamma curves will have different base ISO’s (which is why I prefer to use dB gain as 0dB = native sensitivity, least noise, best dynamic range, no matter what gamma curve).

To get the best possible image we then want to make our signal (picture information) as big as possible. As we can’t swap out the sensor, the only way to do that is to put as much light as possible onto the sensor. Obviously we don’t want to overload the sensor or exceed the limitations of the recording system, but generally the more light you get on the sensor, the better your pictures will be.

As the sensors noise output remains more or less constant, the best signal to noise ratio will be gained when you put a lot of light on the sensor. This generates a very large signal, so the signal becomes big compared to the noise and the noise becomes only a small percentage of the overall image.

If we are unable to get enough light onto the sensor to expose it fully then it is often tempting to add some gain to make the picture brighter. 6dB of gain is the equivalent to 1 stop of exposure. Just like f-stops, each time we go up a stop we are doubling. So adding 6db of gain doubles everything. It makes the picture the equivalent of one stop brighter, but it also doubles the noise. Adding 12dB gain multiplies the noise 4 times, adding 18dB multiplies the noise 8 times.

What if instead of adding gain to make the picture brighter we let 4x more light fall on the sensor (2 stops)? Well the image gets brighter by the equivalent of 2 stops but as we are not adding gain this means the desirable signal, the picture is now going to be the equivalent of 12dB bigger than the noise than it was before we added the 2 stops of light. That’s going to give you a much cleaner looking image.

How do you get more light onto the sensor? There are many ways such as using a faster lens with a larger aperture that will let more light through. Or you could try using a slower shutter speed (I often find it beneficial in low light to use a 1/24th or 1/25th shutter if there is not too much motion to cause the image to become excessively blurred). Then of course you can also add light to your scene by lighting it. It’s very rare to find noisy and grainy night scenes in feature films and that’s because the night scenes normally have well lit foregrounds but keep dark backgrounds to maintain the sensation of night time or darkness. High contrast is the key to good looking night scenes, well lit foregrounds or actors with deep, dark shadows and backgrounds.

The desire to have a good signal to noise ratio is one of the reasons why when shooting in log or raw you want to expose as brightly as you can (while still maintaining consistent exposure from shot to shot, scene to scene). It’s a little bit harder with standard gammas as we have things like the knee or highlight roll off to deal with. Plus the need to have a shot that looks correct straight out of the camera. But at the end of the day the best results are almost always gained when the gain is kept to a minimum (but don’t use negative gain as this can effect the dynamic range) and the amount of light falling on the sensor as high as possible.

In the next article I’ll give you an interesting experiment to try on a PMW-F5, F55 or PMW-FS7 that is very revealing  about the way ISO, gain, exposure and noise behaves that will show why exposing log or raw at +1 to +2 stops is so important.

PXW-FS7 CineEI Guide Updated.

I have just completed a long overdue update to my guide to CineEI and S-Log2/S-Log3 on the PXW-FS7. I have made some changes to reflect the new expanded zebra range available in the latest firmware versions as well as the new waveform operation. I have also added in the two videos I have made on using CineEI. The first video covers setting up the Cine EI mode and how to expose S-Log2/3 and then the second video explains how to use the exposure index function to offset your exposure for less noise and grain.

https://www.xdcam-user.com/2014/12/ultimate-guide-for-cine-ei-on-the-sony-pxw-fs7/

Ultimate Guide for Cine EI on the Sony PXW-FS7

Ultimate Guide to CineEI on the PXW-FS7 (Updated May 2016).

INTRODUCTION:

This guide to Cine-EI is based on my own experience with the Sony PXW-FS7. There are other methods of using LUT’s and CineEI. The method I describe below, to the best of my knowledge, follows standard industry practice for working with a camera that uses EI gain and LUT’s.

If you find the guide useful, please consider buying me a beer or a coffee. It took quite a while to prepare this guide and writing can be thirsty work.


Type



pixel Ultimate Guide for Cine EI on the Sony PXW-FS7

Through this guide I hope to help you get the very best from the Cine EI mode on the PXW-FS7.

The camera has two very distinct shooting mode, Cine EI and Custom Mode. In custom mode the camera behaves much like any other traditional video camera where what you see in the viewfinder is what’s recorded on the cards. In custom mode you can change many of the cameras settings such as gamma, matrix, sharpness etc to create the look you are after in-camera. “Baking-in” the look of your image in camera is great for content that will go direct to air or for fast turn around productions. But a baked-in look can be difficult to alter in post production. In addition it is very hard to squeeze every last drop of the picture information that the sensor can capture in to the recordings in this mode.

The other mode, Cine-EI, is primarily designed to allow you to record as much information about the scene as possible. The footage from the camera becoming, in effect a “digital negative” that can then be developed in post production and the final, highly polished look of the film or video created in post. In addition the Cine-EI mode mimics the way a film camera works giving the cinematographer the ability to rate the camera at different ISO’s to those specified by Sony. This can be used to alter the relative noise levels in the footage or to help deal with difficult lighting situations.

One further “non-standard” way to use Cine-EI is to use a LUT (Look Up Table) to create an in-camera look that can be baked in to the footage while you shoot. This offers an alternative to custom mode. Some users will find it easier to create a specific look for the camera using a LUT than they would by adjusting camera settings such as gamma and matrix.

MLUT’s and LOOK’s (both are types of Look Up Tables) are only available in the Cine-EI mode.

 

 

THE SIMPLIFIED VERSION:

Before I go through all the “why’” and “hows” first of all let me just say that actually, CineEI is easy. I’ve gone in to a lot of extra detail here so that you can fully master the mode and the concepts behind it.

But in it’s simplest form, all you need to do is to turn on the MLUT’s. Choose the MLUT that you like the look of, or is closest to the final look you are after. Expose so that the picture in the viewfinder or on your monitor looks how you want and away you go.

Then in post production bring in your S-log footage. Apply the same LUT as you used when you shot and the footage will look as shot. Or just grade the footage as desired without a LUT, it is not essential to use a LUT in post production.  As the footage you have shot is either raw or Slog you have a huge range of adjustment available to you in post.

THAT’S IT! If you want, it’s that simple (well almost).

If you want to get fancy you can create your own LUT and that’s really easy too (see the end of the document). If you want less noise in your pictures use a lower EI. I shoot using 800EI on my FS7 almost all the time.

Got an issue with a very bright scene and strong highlights, shoot with a high EI (this should only ever be a last resort, try to avoid using an EI higher than 2000EI).

Again, it’s really simple.

But anyway, lets learn more about it and why it works the way it works.

LATITUDE AND SENSITIVITY.

The latitude and sensitivity of the PXW-FS7, like most cameras is primarily governed by the latitude and sensitivity of the sensor. The latitude of the sensor in the FS7 is around 14 stops. Adding different amounts of conventional camera gain or using  different ISO’s does not alter the sensors actual sensitivity to light, only how much the signal from the sensor is amplified. This is like turning up or down the volume on a radio, the sound level gets higher or lower, but the strength of the radio signal is just the same. Turn it up loud and not only does the music get louder but also any hiss or noise, the ratio of signal to noise does not change, so BOTH the noise and the music get louder. Turn it up too loud and it will distort. If you don’t turn it up loud enough, you can’t hear it, but the radio signal itself does not change. It’s the same with a video cameras sensor. It always has the same sensitivity, With a conventional camera, or when the FS7 is in Custom Mode we can add or take away gain (volume control?) to make the pictures brighter or darker (louder?) but the noise levels will go up and down too.

NATIVE ISO:

Sony’s native ISO rating for the FS7 of 2000 ISO has been chosen by Sony to give a good trade off between sensitivity, noise and over/under exposure latitude. In general the native ISO will give excellent results. But there may be situations where you want or need different performance. For example you might prefer to trade off a little bit of over exposure headroom for a better signal to noise ratio, giving a cleaner, lower noise picture. Or you might need a very large amount of over exposure headroom to deal with a scene with lots of bright highlights.

The Cine EI mode allows you to change the effective ISO rating of the camera, without altering the dynamic range.

With film stocks the film manufacturer will determine the sensitivity of the film and give it an Exposure Index which is normally the equivalent of the films measured ASA/ISO.  It is possible for a skilled cinematographer to rate the film stock with a higher or lower ISO than the manufacturers rating to vary the look or compensate for filters and other factors. You then adjust the film developing and processing to give a correctly exposed looking image. This is a common tool used by cinematographers to modify the look of the film, but the film stock itself does not actually change it’s base sensitivity, it’s still the same film stock with the same base ASA/ISO.

Sony’s Cine EI mode and the EI modes on Red and Arri cameras are very similar. While it has many similarities to adding conventional video camera gain, the outcome and effect can be quite different. If you have not used it before it can be a little confusing, but once you understand the way it works it is very useful and a great way to shoot. Again, a key thing to remember that the actual sensitivity of the sensor itself never changes.

 

CONVENTIONAL VIDEO CAMERA GAIN.

Increasing conventional camera gain will reduce the cameras dynamic range as something that is recorded at maximum brightness (109%) at the native ISO or 0db would be pushed up above the peak recording level and we can’t record a signal larger than 109%. But as the true sensitivity of the sensor does not change, the darkest object the camera can actually detect remains the same. Dark objects may appear a bit brighter, but there is still a limit to how dark an object the camera can actually see and this is governed by the sensors noise floor and signal to noise ratio (how much noise there is in the image coming from the sensor).

Any very dark picture information will be hidden in the sensors noise. Adding gain will bring up both the noise and darkest picture information, so anything hidden in the noise at the native ISO (or 0db) will still be hidden in the noise at a higher gain or ISO as both the noise and small signal are amplified by the same amount. So adding gain does not extend the the ability to see further into the shadows, but does decrease the ability to record bright highlights. The net result of adding gain is a decrease in dynamic range.

Using negative gain or going lower than the native ISO may also reduce the dynamic range as picture information very close to black will be shifted down below black when you subtract gain or lower the ISO. At the same time there is a limit to how much light the sensor can deal with before the sensor itself overloads. So even though reducing the ISO or gain may make the picture darker, the sensors clipping/overload point remains the same, so there is no change to the upper dynamic range, just a reduction in recording level. The net result is you loose shadow information, don’t gain any highlight information, this again means a reduction in dynamic range.

See also this article on gain and dynamic range.

As Sony’s Slog2 and Slog3 are tailored to capture the cameras full 14 stop range this means that when shooting with Slog2 or Slog3 the gamma curve will ONLY work as designed and deliver the maximum dynamic range when the camera is at it’s native ISO. At any other recording ISO or gain level the dynamic range will be reduced. IE: If you were to use SLog2 or SLog3 with the camera in custom mode and not use the native ISO by adding gain or changing the ISO away from 2000, you will not get the full 14 stop range that the camera is capable of delivering.

EXPOSURE LEVELS FOR DIFFERENT GAMMA CURVES AND CONTRAST RANGES.

It’s important to understand that different gamma curves with different contrast ranges will require different exposure levels. The TV system that we use today is currently based around a standard known as Rec-709. This standard specifies the contrast range that a TV set or monitor can show and which recording levels represent which display brightness levels. Most traditional TV cameras are also based on this standard. Rec-709 does have some serious restrictions, the brightness and contrast range is very limited as these standards are based around TV standards and technologies developed 50 years ago. To get around this issue most TV cameras use methods such as a “knee” to compress together some of the brighter part of the scene in to a very small recording range.

A traditional TV camera with a limited dynamic range compresses only a small highlight range.

Slide2 Ultimate Guide for Cine EI on the Sony PXW-FS7
A traditional TV camera with a limited dynamic range compresses only a small highlight range.

As you can see in the illustration above only a very small part of the recording “bucket” is used to hold a moderately large compressed highlight range. In addition a typical TV camera can’t capture all of the range in many scenes anyway. The most important parts of the scene, from black to white (such as a white piece of paper), is captured more or less “as is”. This leaves just a tiny bit of space above white to squeeze in a few highly compressed highlights. The black to white range represents about 5 stops, these are the most important stops as the majority of things that are important fall in this range. Faces, skin tones, plants, buildings etc all fall within the black to white range. Anything brighter than white must be a direct light source such as the sky, a reflection or lamp.

The signal from the TV camera is then passed directly to the TV and as the shadows, mid range and skin tones etc are all at more or less the same level as captured the bulk of scene looks OK on the TV/Monitor. Any highlights or other brighter than white such as direct light sources may look a little “electronic” due to the very large amount of compression used.

But what happens if we want to record more of the scenes range or compress the highlights less? As the size of the recording “bucket”, the codec etc, does not change, in order to capture a greater range and fit it in to the same space, we have to re-distribute how we record things.

Recording a greater dynamic range into the same sized bucket.

Slide3 Ultimate Guide for Cine EI on the Sony PXW-FS7
Recording a greater dynamic range into the same sized bucket.

Above you can see instead of just compressing a small part of the highlights we are now capturing the full dynamic range of the scene. To do this we have altered the levels that everything is recorded at. Blacks and shadows are recorded lower, greys and mids are lower and white is a lot lower. By bringing all these levels down, we make room in our recording bucket for the highlights and the really bright stuff without them being excessively compressed.

The problem with this though is that when you output the picture to a monitor or TV it looks odd. It will lack contrast as the really bright stuff is displayed at the same brightness as the conventional 709 highlights. White is now darker then faces would be with a conventional TV camera.

This is how the Hypergammas work:

This is how the Hypergamma works. By re-distributing the recording levels we can squeeze a much bigger dynamic range into the same size recording bucket. But it won’t look right when viewed directly on a standard TV or monitor. It may a little look dark and perhaps a bit washed out. This is because the cameras gamma curve now no longer matches the monitors gamma curve.

I hope you can also see from this that whenever the cameras gamma curve does not match that of the TV/Monitor, the picture might not look quite right. Even when correctly exposed, white may be at different levels, depending on the gamma being used, especially if the gamma curve has a greater range than the normal Rec-709 used in old school TV cameras.

S-Log uses recording levels very different to conventional gammas.

S-Log takes this a step further and instead of using a highlight roll off, knee or other form of highlight compression, S-Log takes every stop that is brighter than middle grey and records each with the same amount of data. This is “log” encoding and is very different to the way a conventional gamma curve works. To fit a big dynamic range into our restricted range recording bucket each of the recorded stops is kept relatively small. Because the way the recording is made and the way the data is distributed is very different to the levels that a normal Rec-709 TV expects, when viewed on a Rec-709 TV, S-log doesn’t look great. It looks flat and lacks contrast. However it is worth understanding that if the TV actually had S-Log as it’s gamma curve the picture would look no different to a picture recorded with normal rec-709, it would not be flat, it would have lots of contrast. The only difference is that it would have a bigger dynamic range, so if the TV could do it, the highlights would be brighter.

THE CORRECT EXPOSURE LEVELS FOR SLOG-2 and SLOG-3.

Before we go any further lets just look at the correct exposure levels for SLog-2 and SLog-3 as recommended by Sony. As these gamma curves have a very large dynamic range the recording levels that they use are very different to the levels used by the normal 709 gamma curve used for conventional TV. As a result when correctly exposed, Slog looks flat and low contrast on a conventional monitor or in the viewfinder. The table below has the correct levels for middle grey (grey card) and 90% reflectance white (a white card or white piece of paper) for the different types of Slog.

log-exposure-1024x190 Ultimate Guide for Cine EI on the Sony PXW-FS7
Correct exposure levels for Sony’s Slog.

Correct exposure levels for Sony’s Slog.

The white level in particular is a lot lower than we would normally use for TV gamma. This is done to give extra space above white in the recording bucket to fit in the extended range that the camera is capable of capturing, all those bright highlights, bright sky and clouds and other things that cameras with a smaller dynamic range struggle to capture.

SETTING THE CORRECT EXPOSURE.

Let’s now take a look at how to set the correct starting point exposure for SLog-3. You can use a light meter if you wish, but if you do want to use a light meter I would first suggest you check the calibration of the light meter by using the grey card method below and comparing what the light meter tells you with the results you get with a grey or white card.

The most accurate method is to use a good quality grey card and a waveform display. For the screen shots seen here I used a Lastolite EzyBalance Calibration Card. This is a pop up grey card/white card that fits in a pocket but expands to about 30cm/1ft across giving a decent sized target. It has middle grey on one side and 90% reflectance white on the other. With the MLUT’s off, set the exposure so that the grey card is exposed at the appropriate level (see table above). If the firmware in your camera is up to date (at least version 3.0) you can set the zebras to 32% or 41% to do this or use an external monitor with a waveform display. The FS7’s built in waveform display is very had to use as it is so small and has no scale. I also recommend the use of a DSC Labs “One Shot” chart. The front of the chart has a series of color references that can be used in post production to set up your base color correction while the rear of the chart has both a large middle grey and 90% white square.

USING THE FS7’s WAVEFORM MONITOR OR ZEBRAS TO SET THE CORRECT BASE S-LOG3 EXPOSURE.

IMPORTANT NOTE: If you use a LUT, The Zebras measure the viewfinder image, so if a LUT is on for the the viewfinder, then the zebras measure the LUT. If there is no viewfinder LUT then the zebras measure the S-Log.

The Waveform Monitor and Histogram measure the SDI2 levels. So if you have a LUT on for SDI2 then the LUT levels are measured. If there is no LUT on SDI2 then the S-Log levels are measured.

See this video for more information on the Waveform, Histogram and Zebras:

The internal waveform display settings are found in the menu under:

VF: Display On/Off: Video Signal Monitor.

LUT-middlegrey41 Ultimate Guide for Cine EI on the Sony PXW-FS7
Setting the correct exposure for Slog-3 using a grey card. Middle grey should be 41%

Setting the correct exposure for Slog-3 using a grey card. Middle Grey should be 41%

If you don’t have access to a better waveform display you can use a white card or grey card and zebras. When using zebras I prefer to use white as the reference as it is easier to see the zebras on a white target than a grey one. By setting up the Zebras with a narrow aperture window of around 3% you can get a very accurate exposure assessment for white. For SLog-3 set the Zebras to 3% aperture and the level at 61%.  For Slog-2 set the zebra level to 59%. To be honest, if you were to set the zebras to 60% this is going to work for both S-Log2 and S-Log3, a 1% error is too small to make any difference and variations in lighting or the white target will be greater than 1% anyway.

Setting up the Zebras to measure S-Log3 exposure of white card (90% reflectance white card).

zebras61 Ultimate Guide for Cine EI on the Sony PXW-FS7
Setting up the Zebras to measure S-Log3 exposure of white card (90% reflectance white card).

Correct exposure for S-Log3 when using a 90% reflectance white target.

LUT-white61 Ultimate Guide for Cine EI on the Sony PXW-FS7
Correct exposure for S-Log3 when using a 90% reflectance white target.

The image above shows the use of both the Zebras and Waveform to establish the correct exposure level for S-Log3 when using a 90% reflectance white card or similar target. Please note that very often a piece of white paper or a white card etc will be a little bit brighter than a calibrated 90% white card. If using typical bleached white printer paper I suggest you add around 4% to the white values in the above chart to prevent under exposure.

This will get you to the base exposure recommended by Sony, without using a LUT. But very often we want to expose brighter than this to improve the signal to noise ratio.

See also the video below for information on how to setup and use S-Log2 and S-Log3 in the CineEI mode:

 

USING LUTS’s and CINE EI:

SO HOW DOES CINE-EI WORK?

FS7-CineEI-seletion-page-1024x576 Ultimate Guide for Cine EI on the Sony PXW-FS7
Selecting Cine EI in base settings on the PXW-FS7

Cine EI is selected in the Base Settings page. It works in YPbPr, RGB and Raw main operation modes.

Cine-EI (Exposure Index) works differently to conventional camera gain. It’s operation is similar in other cameras that use Cine-EI or EI gain such as the F5, F55, F3, F65, Red or Alexa. You enable Cine-EI mode in the camera menus Base Settings page. On the F5 and F55 it works in YPbPr, RGB and RAW modes.

IMPORTANT: In the Cine-EI mode the ISO of the recordings remains fixed at the cameras native ISO (unless baking in a LUT,  more on that later). By always recording at the cameras native ISO you will always have 14 stops of dynamic range.

YOU NEED TO USE A LUT FOR CINE EI TO WORK:

You can only use LUT’s in the CineEI mode. In addition in order to be able to have LUT’s on for the Viewfinder, HDMI / SDI2, but NOT on the SD1 & Internal Rec you cannot set the HDMI to output 4K, you can only use HD or 2K.

FS7-Output-Settings-1024x576 Ultimate Guide for Cine EI on the Sony PXW-FS7
PXW-FS7 output options.

So for most applications you will want to set your SDI and HDMI outputs to HD/2K in order to ba able to use the LUT system as designed for CineEI. For reference (2-3PD) means 2-3 pull down is added for 24p footage, so the output will be 60i with 24p footage sgown using pull down. PsF means progressive segmented frame which is the normal HDSDI standard for progressive output. Any of the HD or 2K output modes will allow the use of LUT’

Important: For Cine-EI mode to work as expected you MUST monitor your pictures in the viewfinder or via the SDI/HDMI output through one of the cameras built in MLUT’s (Look Up Table), LOOK’s or User3D LUT’s. So make sure you have the MLUT’s turned on. If you don’t have a LUT then it won’t work as expected because the EI gain is applied to the cameras LUT’s.

At this stage just set the MLUT’s to on for the Sub&HDMI output and the Viewfinder out.

FS7-LUT-settings-1024x576 Ultimate Guide for Cine EI on the Sony PXW-FS7
PXW-FS7 Lut selection settings.

The LUT’s are then turned on in the VIDEO: Monitor LUT: settings page of the menu. You will normally want to turn ON LUT’s for SDI2, HDMI and the VIEWFINDER (not seen in the image above, simply scroll down to the bottom of the page to see the VIEWFINDER option). For normal CinEI use you should leave SD1 & Internal Rec OFF as we don’t want to record the LUT, just monitor via the LUT.

EXPOSING VIA THE LUT/LOOK.

When viewing or monitoring via a LUT you should adjust your exposure so that the picture in the viewfinder looks correctly exposed. If the LUT is correctly exposed then The S-Log recording will also be correctly exposed. As a point of reference, middle grey for Rec-709 and the 709(800) LUT should be at, or close to 44% and white will be 90%. Skin tones and faces will be at the normal TV level of around 65-70%. As these levels are waht we are used to seeing with a conventional video camera, this makes judging exposure easy.

This is really quite simple, generally speaking when using a Rec709  LUT, if it looks right in the viewfinder, it probably is right. However it is important to note that different LUT’s will have slightly different optimum exposure levels. For example the 709(800) LUT is designed to be a very close match to the 709 gamma curve used in the majority of monitors, so this particular LUT is really simple to use because if the picture looks normal on the monitor then your exposure will also be normal. The included 709(800) LUT is the most accurate LUT for exposure as this matches the gamma used in the majority of monitors. This LUT produces a nice contrasty image that is easy to focus. It is not meant to be pretty! It is a tool to help you get accurate exposure simply and easily.

Correct exposure of Middle Grey for the 709(800) MLUT. Middle Grey should be 44%. 90% white (a white piece of paper) will be 90% and skin tones will be around 65-70%.

Correct exposure of the 709(800) LUT using a 90% white card, white will be 90%. You can use zebras at 90% to check this level (remember zebras etc measure the LUT exposure level when LUT’s are turned on).

LUT-middlegrey42 Ultimate Guide for Cine EI on the Sony PXW-FS7
Correct exposure of Middle Grey for the 709(800) MLUT. Middle Grey should be 42%. 90% will be 90%.
LUT-white90 Ultimate Guide for Cine EI on the Sony PXW-FS7
Correct exposure of the 709(800) LUT using a 90% white card, white will be 90%. You can use zebras at 90% to check this level.

The above images show the correct exposure levels for the 709(800) LUT. Middle grey should be 44% and 90% white is… well 90%. Very simple and you can easily use zebras to check the white level by setting them to 90%. As middle grey is where it normally is on a TV or monitor and white is also where you would expect to see it, when using the 709(800) LUT, if the picture looks right in the viewfinder then it generally is right. This means that the 709(800) LUT is particularly well suited to being used to set exposure as a correctly exposed scene will look “normal” on a 709 TV or monitor. SIMPLE!

I don’t recommend the use of any of the other LUT’s to set exposure because all of the other LUT’s have brightness ranges that are different to Rec-709. As a result the LUT has to be exposed at non standard levels to ensure the S-Log is exposed correctly. You can use any of the other LUT’s or LOOK if you really wish, but you will need to figure out the correct exposure levels for each LUT.

The LC709-TypeA Look is very popular as a LUT for the FS7 as it closely mimics the images you get from an Arri Alexa (“type A” = type Arri).

The “LC” part of the Look’s name means Low Contrast and this also means – big dynamic range. Whenever you take a big dynamic range (lots of shades) and show it on a display with a limited dynamic range (limited shades) all the shades in the image get squeezed together to fit into the monitors limited range and as a result the contrast gets reduced. This also means that middle grey and white are also squeezed closer together. With conventional 709 middle grey would be 42% and white around 80-90%, but with a high dynamic range/low Contrast gamma curve white gets squeezed closer to grey to make room for the extra dynamic range. This means that middle grey will remain close to 42% but white reduces to around 72%. So for the LC709 Looks in the FS7 optimum exposure is to have middle grey at 42% and white at 72%. Don’t worry too much if you don’t hit those exact numbers, a little bit either way does little harm.

Correct white level for the LC709 LOOK’s. White should be around 72%

LUT-white72 Ultimate Guide for Cine EI on the Sony PXW-FS7
Correct white level for the LC709 LOOK’s. White should be around 72%

Top Tip: Not sure how many people are aware of this function and how it works, but it’s a great way to get around the inability to easily turn the LUT’s on and off in the CineEI mode.

Assign the Hi/Low Key option to one of your assignable buttons. So when using the 709(800) LUT (or any other LUT for that matter) the first press of the button darkens the VF image so you can see what highlights beyond the range of the LUT are doing exposure wise. This allows you to check for clipping that may be present in the much wider range S-log recordings. Press it again and you will see the image brighten allowing you to see further into the shadows so you can see the darkest things being captured by the S-log recordings. The Hi/Low Key function is a great way of seeing your full available exposure range without needing to turn the LUT on and off.

LUT EXPOSURE LEVELS FOR THE OTHER LUTS.

Here are some white levels for some of the built in LUT’s. The G40 or G33 part of the HG LUT’s is the recommended value for middle grey. Use these levels for the zebras if you want to check the correct exposure of a 90% reflectance white card. I have also include an approximate zebra value for a piece of typical white printer paper.

709(800) = Middle Grey 42%. 90% Reflectance white 90%, white paper 92%.

HG8009(G40) = Middle Grey 40%. 90% Reflectance white 83%, white paper 86%.

HG8009(G33) = Middle Grey 33%. 90% Reflectance white 75%, white paper 80%.

The “LC709” LOOK’s = Middle Grey 42%. 90% Reflectance white 72%, white paper 77%.

DONT PANIC if you don’t get these precise levels! I’m giving them to you here so you have a good starting point. A little bit either way will not hurt. Again, generally speaking if it looks right in the viewfinder or on your monitor screen, it is probably close enough not to worry about it.

BUT, again I would suggest sticking to the 709(800) LUT for setting exposure. It’s not the prettiest LUT, but is the only one of the included LUT’s that gives the correct, normal, brightness and contrast range on a conventional monitor, viewfinder or TV. If you want to keep things simple and accurate use 709(800).

USING EI OR EXPOSURE INDEX.

What is EI? EI stands for Exposure Index. This is NOT the same thing as ISO.

ISO is the sensitivity of the camera. ISO is the sensitivity that the camera records at.

EI is the sensitivity of the LUT. EI is the brightness at which the LUT displays the scene.

The FS7 has a native ISO of 2000 and the camera always records at 2000 ISO in the Cine EI mode.

But the EI of the LUT can be varied to make the LUT brighter and darker. the only thing EI changes is the brightness of the LUT. But when exposing via the LUT, if the LUT is made darker, to compensate for the dark looking LUT you open the aperture to let in more light. This will make the LUT look correct again. It will also result in a recording that is brighter than normal as we have opened the aperture.

CHANGING THE EI.

Latitude Indication.

At the native 2000 EI you have 6 stops of over exposure latitude and 8 stops of under exposure latitude (6 stops above middle grey and 8 stops below middle grey). This is how much headroom your shot has. Your over exposure latitude is indicated whenever you change the EI level. In the image below you can see the EI 2000EI followed by a 6.0E the 6.0E is the over exposure latitude.

FS7-EI-indication-2-1024x576 Ultimate Guide for Cine EI on the Sony PXW-FS7
The EI and Lattitude indication on the FS7.

The EI gain is altered by changing the cameras gain switch and the EI levels assigned to each of the Hi/Mid/Low switch positions can be changed in the camera menu. I recommend setting the EI steps to H 2000, M 1000 and L 500 as this allows you to select the native EI plus 1 stop and 2 stops down (each time you halve the ISO you are shifting the exposure one stop down).

FS7-ISO-settings-page-1024x576 Ultimate Guide for Cine EI on the Sony PXW-FS7
The PXW-FS7 EI settings for the gain switch.

REDUCING THE EI.

So what happens when you halve the EI gain to 1000EI?  1 stop of gain/ISO will subtracted from the LUT. As a result the picture you see via the LUT becomes one stop darker (a good thing to know is that 1 stop of exposure is the same as 6db of gain or a doubling or halving of the ISO). So the picture in the viewfinder gets darker. But also remember that the camera will still be recording at the native ISO (unless baking-in the LUT).

 

 

Why does this happen and what’s happening to my pictures?

First of all lets take a look at the scene, as seen in the cameras viewfinder when we are at the native 2000 EI and then with the EI changed one stop down so it becomes 1000EI. The native ISO on the left, the one stop lower EI on the right.

VF-side-by-side Ultimate Guide for Cine EI on the Sony PXW-FS7
2000EI and 1000EI as seen in the viewfinder with NO exposure change.

2000EI and 1000EI as seen in the viewfinder with NO exposure change.

So, in the viewfinder, when you lower the EI by one stop (halving the EI) the picture becomes darker by 1 stop. If using an external monitor with a waveform display connected to SDI2 or the HDMI output this too would get darker and the waveform levels decrease by one stop.

As a camera operator, what do you do when you have a dark picture? Well most people would normally compensate for a dark looking image by opening the iris to compensate. As we have gone one stop darker with the EI gain, making the LUT 1 stop darker, to return the viewfinder image back to the same brightness as it was at the native EI you would open the iris by one stop.

So now, after reducing the EI by one stop and then compensating by opening the iris by 1 stop, the viewfinder image is the same brightness as it was when we started.

But what’s happening to my recordings?

Remember the recordings, whether on the XQD card (assuming the SD1 & Internal Rec LUT is OFF) are always at the cameras native 2000 ISO, no matter what the EI is set to. As a result, because you will have opened the iris by 1 stop to compensate for the dark viewfinder image the recording will have become 1 stop brighter. Look at the image below to see what we see in the viewfinder alongside what is actually being recorded. The EI offset exposure with aperture correction as seen in the viewfinder (left hand side) looks normal, while the actual native ISO recording (right hand side) is 1 stop brighter.

At 1000EI the Viewfinder image on the left is 1 stop darker than the actual recorded image (on the right) which is recorded at the native 2000 ISO.

VF-and-Internal Ultimate Guide for Cine EI on the Sony PXW-FS7

How does this help us, what are the benefits?

When you take this brighter recorded image in to post production the colorist will have to bring the levels back down to normal as part of the grading process. As he/she will be reducing the levels in post production by around 1 stop (6db) any noise in the picture will also be reduced by 6db. The end result is a picture with 6db less noise than if shot at the native EI. Another benefit may be that as the scene was exposed brighter you will be able to see more shadow information.

Is there a down side to using a low EI?

Because the actual recorded exposure is brighter by one stop you have one stop less headroom. However the PXW-FS7 has an abundance of headroom so the loss of one stop is often not going to cause a problem. I find that going between 1 and 1.5 EI stops down rarely results in any highlight issues. But when shooting very high contrast scenes and using a low EI it is worth toggling the LUT on and off to check for clipping in the SLog image.

It’s also worth noting the S-Log does not have a highlight roll off. Each stop above middle grey is recorded with the same amount of data, so exposing brighter by a stop or two does not alter the contrast as it would with a standard gamma. So over exposing log is NOT a bad thing. It is in fact in most cases highly beneficial.

Log gamma curves have very little picture information in the shadows, so if we can expose brighter our shadows will look much better. 

What is happening to my exposure range?

What you are doing is moving the mid point of your exposure range up in the case of a lower EI (up because you are opening the aperture, thus making the recordings brighter). This allows the camera to see deeper into the shadows increasing the under exposure latitude, but reduces the over exposure latitude. The reverse is also possible. If you use a higher EI you shift your mid point down. This gives you more headroom for dealing with very bright highlights, but you won’t see as far into the shadows and the final pictures will be a little noisier as in post production the overall levels will have to be brought up to compensate for the darker overall recordings.

Cine-EI allows us to shift our exposure mid point up and down.  Lowering the EI gain gives you a darker VF image so you compensate by opening the aperture which results in brightly exposed footage. This reduces over exposure headroom but increases under exposure range (and improves the signal to noise ratio). Adding EI gain gives a brighter Viewfinder image which makes you expose the recordings darker, which gives you more headroom but with less underexposure range (and a worse signal to noise ratio).

When shooting with almost any CineEI camera I will use an EI that is between 1 and 2 stops darker than the base settings. So on the FS7 I normally set the EI to 800 EI. It’s very rare to get any highlight problems at 800 EI and the improvement this low EI brings to the noise levels in the image is very nice.

Slide01 Ultimate Guide for Cine EI on the Sony PXW-FS7

Post Production.

When shooting raw information about the EI gain is stored in the clips metadata. The idea is that this metadata can be used by the grading or editing software to adjust the clips exposure level in the edit or grading application so that it looks correctly exposed (or at least exposed as you saw it in the viewfinder via the LUT). The metadata information is recorded alongside the XAVC footage when shooting SLog2/3. However, currently few edit applications or grading applications use this metadata to offset the exposure, so S-Log2/3 material may look dark/bright when imported into your edit application and you may need to add a correction to return the exposure to a “normal” level. You can use a correction LUT to move the exposure up and when I provide LUT sets on this website I will always try to include LUT’s for over and under exposure. Another way to deal with brightly exposed log footage in post production is to first apply an “S” curve using the luma curve tool to the log. Then a simple gain adjustment will shift the exposure.

See this video for detailed information on how to expose using CineEI:

 

 WHAT IF YOU ARE SHOOTING USING HFR (High Frame Rate) AND LUT’S CANT BE USED.

In HFR you can either have LUT’s on for everything including internal recording, or all off, no LUT’s at all. This is not helpful if your primary recordings are internal S-Log.

So for HFR in many cases you will have to just work viewing the native S-log. If you set zebras to 70% and expose a white card at 70% this will result in S-Log footage that is 1.2 – 1.5 stops over exposed. This is the same as shooting at 800 EI and I highly recomend this approach for HFR (slow motion) shooting as it will help clean up the additional noise that you see when shooting HFR.

BAKING IN THE LUT/LOOK.

When shooting using a high or low EI, the EI gain is added or subtracted from the LUT or LOOK, this makes the picture in the viewfinder or monitor fed via the LUT brighter or darker depending on the EI used. In Cine-EI mode you want the camera to always actually record the S-log at the cameras native 2000 ISO. So normally you want to leave the LUT’s OFF for the internal recording. Just in case you missed that very important point: normally you want to leave the LUT’s OFF for the internal recording!

FS7-Lut-settings-2-1024x576 Ultimate Guide for Cine EI on the Sony PXW-FS7
You need to turn ON the SD1 and Internal Rec LUT t “Bake In” a LUT. Normally leave this OFF.

Just about the only exceptions to this might be when shooting raw or when you want to deliberately record with the LUT/Look baked in to your XQD recordings. By “baked-in” I mean with the gamma, contrast and color of the LUT/Look permanently recorded as part of the recording. You can’t remove this LUT/look later if it’s “baked-in”.

No matter what the LUT/Look settings, if you’re recording raw on an external raw recorder, recorder the raw is always recorded at 2000 ISO.  But the internal XQD recordings are different. It is possible, if you choose, to apply a LUT/LOOK to the XQD recordings by setting the “SDI1 & Internal Rec” LUT to ON. The gain of the recorded LUT/LOOK will be altered according to the CineEI gain settings. This might be useful to provide an easy to work with proxy file for editing, with the LUT/LOOK baked-in while shooting raw. Or as a way to create an in-camera look or style for material that won’t be graded. Using a baked-in LUT/LOOK for a production that won’t be graded or only have minimal grading is an interesting alternative to using Custom Mode that should be considered for fast turn-around productions.

In most cases however you will probably not have a LUT applied to your primary recordings. If shooting in S-Log you must set LUT – OFF for “SDI1 & Internal Rec” See the image above. With “SDI1 & Internal Rec” Off the internal recordings, without LUT, will be SLog2 or Slog3 and at 2000 ISO.

You can tell what it is that the camera is actually recording by looking in the viewfinder. At the center right side of the display there is an indication of what is being recorded on the cards. Normally for Cine-EI this should say either SLog2 or Slog3. If it indicates something else, then you are baking the LUT in to the internal recordings.

LUT-Slog3-indication Ultimate Guide for Cine EI on the Sony PXW-FS7
The internal recording gamma is shown on the right of the VF. This is recording SLog-3
LUT-LUT709-indication Ultimate Guide for Cine EI on the Sony PXW-FS7
The indication here shows that the 709(800) LUT is being baked-in to the internal recordings.

CINE-EI SUMMARY:

CineEI allows you to “rate” the camera at different ISO.

You MUST use a LUT for CineEI to work as designed.

A low EI number will result in a brighter exposure which will improve the signal to noise ratio giving a cleaner picture or allow you to see more shadow detail. However you will loose some over exposure headroom.

A high EI number will result in a darker exposure which will improve the over exposure headroom but decrease the under exposure range. The signal to noise ratio is worse so the final picture may end up with more noise.

A 1D LUT will not clip and appear to overexpose as readily as a 3D LOOK when using a low EI, so a 1D LUT may be preferable.

When viewing via a 709 LUT you expose using normal 709 exposure levels. Basically if it looks right in the viewfinder or on the monitor (via the 709 LUT) it almost certainly is right.

When I shoot with my FS7 I normally rate the camera at between 800 and 1000EI. I find that 5 stops of over exposure range is plenty for most situations and I prefer the decrease in noise in the final pictures. But please, test and experiment for yourself.

 

 

QUICK GUIDE TO CREATING YOUR OWN LOOK’s (Using DaVinci Resolve).

It’s very easy to create your own 3D LUT for the FS7 using DaVinci Resolve or just about any grading software with LUT export capability. The LUT should be a 17x17x17 or 33x33x33 .cube LUT. This is what Resolve creates by default and .cube LUT’s are the most common types of LUT in use today.

First simply shoot some test Slog3 clips at 2000EI. In addition you should also use the same color space (S.Gamut or S.Gamut3.cine) for the test shot as you will when you want to use the LUT. I recommend shooting a variety of clips so that you can asses how the LUT will work in different lighting situations.

Import and grade the clips from the test shoot in Resolve creating the look that you are after for your production or as you wish your footage to appear in the viewfinder of the camera. Then once your happy with the look of the graded clip, right click on the clip in the timeline and “Export LUT”. Resolve will then create and save a .cube LUT.

Then place the .cube LUT file created by the grading software on an SD card in the PMWF55_F5 folder. You may need to create the following folder structure on the SD card. So first you have a PRIVATE folder, in that there is a SONY folder and so on.

PRIVATE   :   SONY   :    PRO   :   CAMERA   :    PMWF55_F5

Put the SD card in the camera, then go to the “File” menu and go to “Monitor 3D LUT” and select “Load SD Card”. The camera will offer you a 1 to 4 destination memory selection. Choose 1,2,3 or 4, this is the memory location where the LUT will be saved. You should then be presented with a list of all the LUT’s on the SD card. Select your chosen LUT to save it from the SD card to the camera.

Once loaded in to the camera when you choose 3D User LUT’s you can select between user LUT memory 1,2,3 or 4. Your LUT will be in the memory you selected when you copied the LUT from the SD card to the camera.

How to use S-Log3 on the Sony PXW-FS7.

So with the FS7 now shipping and the first units landing in peoples hands I have put together a comprehensive guide to using S-Log3 and CineEI on the PXW-FS7. Please follow this link to read or download the guide to CineEI on the PXW-FS7.

It’s important to note that S-Log3 has a peak recording level of 92IRE so never goes above this. Don’t be surprised to find that your overall levels are going to be much lower than you would normally use for conventional 709 shooting. In addition I can’t stress enough how important it is to learn how to use LUT’s (look up tables) in camera and in post production with this camera. It will make your life so much simpler and easier. LUT’s may sound complicated and difficult, but they are not. If you want to create your own LUT’s take a look at this guide here.

The FS7 is an incredibly powerful camera. But if you really want to get the most from the Cine-EI mode and S-Log then you need to adjust the way you shoot. You can’t just apply normal Rec-709 exposure levels to S-Log3, it’s not designed to work that way. However by using the 709(800) LUT on the viewfinder output you can expose based on the viewfinder image as you would normally, while the S-Log3 recordings will be at the correct levels. So do learn how to implement LUT’s correctly, it will make your life so much easier. Take a look at this video for an idea of how it works. The video features an F5 but the FS7 is the same.

While you’re at it you might also want to take a look at this article on the S-log3 gamma curve. Many people will look at the S-log and think that it looks noisy and be worried by this. You shouldn’t be. The shape of the log curve means that before grading and application of a LUT it can emphasise noise. However once you use a LUT to convert from S-log3 to 709 you will find that most of the noise will go away. Again, please use a LUT as simply trying to grade S-log3 in to 709 space is often not as effective as adding the right LUT. If you really know what you are doing, by using S-Curves and log grading tools it is possible to grade the native S-log3 in a 709 environment, but LUT’s do make it simpler. Another useful way to get from S-log3 to 709 is to use the new color chart tool in Resolve which recognises and corrects either a Macbeth chart or DSC One Shot chart to the correct levels automatically. When you set up this process in Resolve you will select the source gamma as S-Log3 so the correction compensates for the gamma curve as well as adding color correction. I’lll write this up in more depth in the next couple of weeks.

So enjoy your FS7 if you have one. As soon as mine arrives I will write up the correct way (or at least the designed way) to use the Cine-EI mode, in the mean time the F5/F55 Cine-EI guide can be used, the process is exactly the same on the FS7.

Exposing and Using S-Log2 on the Sony A7s. Part One: Gamma and Exposure.

This guide is for the A7S and A7SII. The A7SIII does not in my opinion need the same degree of over exposure as documented here. So please bear this in mind if using the A7SIII.

This document has been prepared independently of Sony. It is based on my own findings having used the camera and tested various exposure levels and methods. Part 2 which explains how to use LUT’s to correct the footage in the edit suite or post production is here: https://www.xdcam-user.com/2014/10/using-s-log2-from-the-a7s-in-post-production/

If you find this useful please consider buying me a coffee or a beer. I’m not paid to write these articles.


Type



pixel Exposing and Using S-Log2 on the Sony A7s. Part One: Gamma and Exposure.

One of the really nice features of the Sony A7s and Sony’s other Alpha cameras, including the A6300, A6500 etc is the ability to use different gamma curves and in particular the Sony S-Log2 gamma curve.

What are gamma curves?

All conventional cameras use gamma curves. The gamma curve is there to make the images captured easier to manage by making the file size smaller than it would be without a gamma curve. When TV was first developed the gamma curve in the camera made the signal small enough to be broadcast by a transmitter and then the gamma curve in the TV set (which is the inverse of the one in the camera) expanded the signal back to a normal viewing range. The current standard for broadcast TV is called “Recommendation BT-709”, often shortened to Rec-709. This gamma curve is based on standards developed over 60 years ago and camera technology has advanced a lot since then! Even so, almost every TV and monitor made today is made to the Rec-709 standard or something very similar. Many modern cameras can capture a brightness range, also known as dynamic range, that far exceed the Rec-709 standard.

The limitations of standard gammas.

As gamma effects the dark to light range of the image, it also effects the contrast of the image. Normal television gamma has a limited dynamic range (about 6 to 7 stops) and as a result also has a limited contrast range.

Normal-Gamma Exposing and Using S-Log2 on the Sony A7s. Part One: Gamma and Exposure.
When shooting a high contrast scene with conventional gamma the brightest highlights and the darkest shadows cannot be recorded. The contrast on the TV or monitor will however be correct as the camera captures the same contrast range as the monitor is able to display.

Normally the gamma curve used in the camera is designed to match the gamma curve used by the TV or monitor. This way the contrast range of the camera and the contrast range of the display will be matched. So the contrast on the TV screen will match the contrast of the scene being filmed and the picture will look “normal”. However the limited dynamic range may mean that very bright or very dark objects cannot be accurately reproduced as these may exceed the gammas dynamic range.

Slide5 Exposing and Using S-Log2 on the Sony A7s. Part One: Gamma and Exposure.
Although the dynamic range of Rec-709 may not always capture the entire range of the scene being shot, as the gamma of the camera matches the gamma of the TV the contrast will appear correct.

The over exposure typical of a restricted range gamma such as Rec-709  is commonly seen as bright clouds in the sky becoming white over exposed blobs or bright areas on faces becoming areas of flat white. Objects in shade or shadow areas of the scene are simply too dark to be seen. But between the overexposed areas and any under exposure the contrast looks natural and true to life.

Slog-709-FH Exposing and Using S-Log2 on the Sony A7s. Part One: Gamma and Exposure.
Typical limited Rec-709 exposure range. Contrast is good but the clouds are over exposed and look un-natural.

Log Gamma.

Log gamma, such as Sony’s S-Log2, allows the camera to capture a much greater brightness range or dynamic range than is possible when shooting with conventional television gamma. Dynamic range is the range from light to dark that the camera can capture or the range that the monitor or TV can display within one image. It is the range from the deepest blacks to the brightest whites that can be captured or shown at the same time.

There are some things that need to be considered before you get too excited about the possibility of capturing this much greater dynamic range. The primary one being that if the camera is set to S-log2 and the TV or monitor is a normal Rec-709 TV (as most are) then there is no way the TV can correctly display the image being captured, the TV just doesn’t have the range to show everything that the camera with it’s high range log gamma can capture accurately.

Fixed Recording Range For Both Standard and Log Gamma.

The signal range and signal levels used to record a video signal are normally described in percent. Where black is 0% and the brightest thing that can be recorded is normally recorded at 100% to 109%. Most modern video cameras actually record the brightest objects at 109%. The important thing to remember though is that the recording range is fixed. Even when you change gamma curve the camera is still constrained by the zero to 109% recording range. The recording range does not change whether you are recording Rec-709 or S-log2. So log gamma’s like S-Log2 must squeeze a much bigger signal range into the same recording range as used by conventional Rec-709 recordings.

Slide6 Exposing and Using S-Log2 on the Sony A7s. Part One: Gamma and Exposure.
Log gamma squeezes the scenes large range to fit in the camera’s normal 0%-109% recording range.

Recording S-Log2.

In order to record using S-log2 with the A7s you need to use a picture profile. The picture profiles give you several recording gamma options. For S-log2 you should use Picture Profile 7 which is already set up for S-log2 and S-Gamut by default (for information on gamuts see this article). In addition you should ALWAYS use the cameras native ISO which is 3200 ISO and it is normally preferable to use a preset white balance. Using any other ISO with S-log2 will not allow you to get the full benefit of the full 14 stops of dynamic range that S-log2 can deliver. In most of the Alpha cameras you now also have the ability to use a different version of S-log, – S-Log3 and this is found in picture profiles 8 and 9. You can use S-Log3 if you wish, but S-Log2 was designed from the outset by Sony to work with digital camera sensors. S-Log3 is based on an older curve designed for film transfers to a 10 bit recording. As a result when using a camera that only has 8 bit recording with a limited number of code values, S-Log2 tends to be more efficient and yield a better end result. This is what it was designed for.

Grey Cards and White Cards.

Before I go further let me introduce you to grey and white cards in case you have not come across them before. Don’t panic you don’t have to own one, although I would recommend getting a grey card such as the Lastolite EzyBalance if you don’t have one. But it is useful to understand what they are.

The 90% White Card.

The 90% white card is a card or chart that reflects 90% of the light falling on it. This will be a card that looks very similar in brightness to a piece of ordinary white paper, it should be pure white, some printer papers are bleached or coloured very slightly blue to make them appear “brilliant white”  (as you will see later in many cases it is possible to use an ordinary piece of white paper in place of a 90% white card for exposure).

The Grey Card.

The 18% grey card, also often called “middle grey” card, is a card that reflects 18% of the light falling on it. Obviously it will appear much darker than the white card. Visually to us humans an 18% grey card appears to be half way between white and black, hence it’s other name, “middle grey”.

Middle grey is important because the average brightness level of most typical scenes tends to be around the middle grey brightness value. Another key thing about middle grey is that because it falls in the middle of our exposure range it makes it a very handy reference level when measuring exposure as it is less likely to be effected by highlight compression than a 90% white card.

Exposing White and Middle Grey.

Coming back to Rec-709 and conventional TV’s and monitors. If we want a piece of white paper to look bright and white on a TV we would record it and then show it at somewhere around 85% to 95% of the screens full brightness range. This doesn’t leave much room for things brighter than a white piece of paper! Things like clouds in the sky, a shiny car, a bright window or a direct light source such as a lamp or other light.  In order to make it possible for S-log2 to record a much greater dynamic range the recording level for white and mid tones is shifted down. Instead of recording white at 85%-95%, when using S-log2 or S-Log3 it is recommended by Sony that white is recorded at around 60%. For S-Log2 Middle grey moves down too, instead of being recorded at 42%-43% (the normal level for Rec-709) it’s recorded at just 32% with S-Log2 (S-log3 uses 41%).

By recording everything white (ie a white piece of paper) and darker in a  lower range, we free up  lot of extra space above the white recording level, within the full recording range, to record all those bright highlights in any scene that would be impossible to record with conventional gammas where there is only 10% to 20% from white at 90% to the peak of the recording range at 100 to 109%.

Slide7 Exposing and Using S-Log2 on the Sony A7s. Part One: Gamma and Exposure.
To make room for the extra dynamic range and the ability to record very bright objects, white and mid tones are shifted down in level by the S-log2 gamma curve. As a result, white, mid tones etc will be displayed darker than normally expected with conventional gamma.

As S-Log2 and S-Log3 normally shift a lot of the recording levels downwards, if we show a scene shot with S-Log2 or S-log3 that has been exposed correctly on a conventional TV or monitor it will look dark due to the lower recording levels. In addition it will look flat with very low contrast as we are now squeezing a much bigger dynamic range into the limited conventional Rec-709 display range of a normal TV or computer monitor.

Slide8 Exposing and Using S-Log2 on the Sony A7s. Part One: Gamma and Exposure.
The on screen contrast appears reduced as the capture contrast is greater than the display contrast.

This on screen reduction in contrast and the darker levels are actually perfectly normal when shooting using log gamma, this is how it is supposed to look on a normal monitor or TV. So don’t be alarmed if when shooting using S-Log your images look a little darker and flatter than perhaps you are used to when shooting with a standard gamma. You will adjust the S-Log footage in post production to restore the brightness and contrast later.

S-log2-correct-FH-1024x576 Exposing and Using S-Log2 on the Sony A7s. Part One: Gamma and Exposure.
Correctly exposed S-Log2 can look dark and washed out.

The post production adjustment of S-Log2 and S-log3 is very important and one of the keys to getting the very best finished images. The S-Log recording acts as a digital negative and by “processing” this digital negative in post production (normally referred to as “grading”) we manipulate the large 14 stop dynamic range of the captured image to fit within the limited display range of a Rec-709 TV in a pleasing manner. This may mean pulling up the mid range a bit, pulling down the highlights and bit and generally shifting the brightness and colour levels of different parts of the image around  (see PART 2 for more post production information).

SLog-2 and 10 bit or 8 bit data.

Originally Slog-2 was designed for use on high end digital cinema cameras such as Sony’s F65 camera. These cameras have the ability to record using 10 bit data. A 10 bit recording can have up to around 1000 shades of grey from black to white. The A7s however uses 8 bit recording which only has a maximum of 235 shades from black to white. Normally 8 bit recording is perfectly OK as most transmission and display standards are also 8 bit. Shoot with an 8 bit camera and then display that image directly via an 8 bit system and nothing is lost. However when you start to grade and manipulate the image the difference between 8 bit and 10 bit becomes more significant. If you start to shift levels around, perhaps stretching out some parts of the image then the increased tonal resolution of a 10 bit recording helps maintain the very highest image quality. Photographers that have shot using both jpeg and raw will know how much more flexibility the 12 bit (or more) raw files have compared to the 8 bit jpeg’s. However they will also know that 8 bit jpeg’s can be also adjusted, provided you don’t need to make very large adjustments.

Contrary to popular belief heavy grading of 8 bit footage does not necessarily lead to banding in footage across smooth surfaces except in extreme cases. Banding is more commonly a result of compression artefacts such as macro blocking. This is especially common with very highly compressed codecs such as AVCHD. The 50Mbps XAVC-S codec used in the Sony Alpha cameras is a very good codec, far superior to AVCHD and as a result compression artefacts are significantly reduced, so banding will be less of an issue than with other lower quality codecs. If you’re going to shoot using S-Log2, some grading will be necessary and as we only have 8 bit recordings we must take care to expose our material in such a way as to minimise how far we will need to push and pull the material.

Getting Your Exposure Right.

When S-Log2 was developed the engineers at Sony produced tables that specified the correct exposure levels for s-Log2 which are:

exposure-table1 Exposing and Using S-Log2 on the Sony A7s. Part One: Gamma and Exposure.As you can see the nominal “correct” exposure for S-Log2 is a lot lower than the levels used for display on a typical Rec-709 TV or monitor. This is why correctly exposed s-log2 looks dark on a conventional TV. The implication of this is that when you grade your footage in post production you will have to shift the S-log2 levels up quite a long way. This may not be ideal with an 8 bit codec, so I decided to carefully test this to determine the optimum exposure level for the A7s.

Correct Exposure.

The panel of images below is from the A7s recording S-log2 and exposed at the Sony recommended “correct” 32% middle grey level. The correct exposure was determined using a grey card and an external waveform monitor connected to the cameras HDMI output. Then the S-log2 was corrected in post production to normal Rec-709 levels using a Look Up Table (LUT – more on LUT’s in part 2). You can also see the viewfinder display from the camera. If you click on the image below you can expand it to full size. Sorry about the shadow from the laundry line, I didn’t see this when I was shooting the test shots!

Slog2-correct-exposure-panel-1024x564 Exposing and Using S-Log2 on the Sony A7s. Part One: Gamma and Exposure.
Correctly exposed S-Log2 from A7s.

From this you can see just how dark and low contrast looking the original correctly exposed S-log2 is and how much more vibrant the corrected Rec-709 image is. I have also indicated where on the cameras histogram middle grey and white are. Note how much space there is to the right of white on the histogram. This is where the extra highlight or over exposure range of S-log2 can be recorded. When correctly exposed S-log2 has an exposure range of 6 stops above middle grey and 8 stops under.

Over Exposing or “Pushing” S-log2.

If we deliberately raise the exposure level above the Sony recommended levels (known as pushing the exposure), assuming you grade the image to the same final levels some interesting things happen.

For each stop we raise the exposure level you will have 1 stop (which is the same as 6db) less noise. So the final images will have half as much noise for each stop up you go. This is a result of exposing the image brighter and as a result not needing to raise the levels in post as far as you would if exposed at the normal level.

You will loose one stop of over exposure headroom, but gain one stop of under exposure headroom.

Bright highlights will be moved upwards into the most compressed part of the log gamma curve. This can result in a loss of texture in highlights.

Skin tones and mid tones move closer to normal Rec-709 levels, so less manipulation is need for this part of the image in post production.

This last point is important for the A7s with it’s 8 bit codec, so this is the area I looked at most closely. What happens to skin tones and textures when we raise the exposure?

Exposing at +1, +2 and +3 Stops.

Below are another 3 panels from the A7s, shot at +1 stop, +2 stops and +3 stops. Again you can click on the images if you wish to view them full size.

Slog2-plus1-exposure-panel-1024x564 Exposing and Using S-Log2 on the Sony A7s. Part One: Gamma and Exposure.
A7s S-Log2 over exposed by one stop.
Slog2-plus2-exposure-panel-1024x564 Exposing and Using S-Log2 on the Sony A7s. Part One: Gamma and Exposure.
A7s S-Log2 over exposed by 2 stops.
Slog2-plus3-exposure-panel-1024x564 Exposing and Using S-Log2 on the Sony A7s. Part One: Gamma and Exposure.
A7s S-Log2 over exposed by 3 stops.

Looking at these results closely you can see that when you increase the exposure by 1 stop over the Sony specified correct level for S-log2 there is a very useful reduction in noise, not that the A7s is particularly noisy to start with, but you do get a noticeably cleaner image.

Below are 4 crops from the same images, after grading. I really recommend you view these images full size on a good quality monitor. Click on the image to view larger or full size.

A7s-over-exposure Exposing and Using S-Log2 on the Sony A7s. Part One: Gamma and Exposure.
Crops at different exposure of LUT corrected A7s S-log2 footage.

The noise reduction at higher exposures compared to the base exposure is very clear to see if you look at the black edge of the colour checker chart (the coloured squares), although the difference between +2 and +3 stops is very small. You can also see further into the shadows in the +3 stop image compared to the base exposure. A more subtle but important effect is that as the exposure goes up the visible texture of the wooden clothes peg decreases. The grain can be clearly seen at the base level but by +3 stops it has vanished. This is caused by the highlights creeping into the more compressed part of the log gamma curve. The same thing is happening to the skin tones in the +3 stop image, there is some reduction of the most subtle textures.

From this we can see that for mid tones and skin tones you can afford to expose between 1 and 2 stops above the Sony recommended base level. More than 2 stops over and brighter skin tones and any other brighter textures start to be lost. The noise reduction gain by shooting between one and 2 stops over is certainly beneficial. The down side to this though is that we are reducing the over amount of exposure headroom.

Slide02 Exposing and Using S-Log2 on the Sony A7s. Part One: Gamma and Exposure.
As you raise the exposure level you reduce the over exposure headroom.

Given everything I have seen with this 8 bit and almost every other 8 bit camera my recommendation is to shoot between the Sony recommended base S-log2 level and up to two stops over this level. I would try to avoid shooting more than 2 stops over as this is where you will start to see some loss of texture in brighter skin tones and brighter textures.  Exactly where you set your exposure will depend on the highlights in the scene. If you are shooting a very bright scene you will possibly need to shoot at the Sony recommended level to get the very best over exposure headroom. If you are able to expose higher without significantly compromising any highlights then you should aim to be up to 2 stops over base. But whatever you do never expose darker than the Sony base level, this will normally look really nasty.

Determining The Correct Exposure.

The challenge of course is determining where your exposure actually is. Fortunately as we have seen, provided you in the right ball park, S-log2 is quite forgiving, so if you are a little bit over exposed it’s probably not going to hurt your images much. If you have a waveform monitor then you can use that to set your exposure according to the table below. If you don’t have proper white or grey cards you can use a piece of normal white paper. Although slightly less accurate this will get you very close to where you want to be. Do note that white paper tends to be a little brighter than a dedicated 90% reflectivity white card. If you don’t have any white paper then you can use skin tones, again a bit less accurate but you should end up in the right zone.

A7s-exposure-levels-1024x358 Exposing and Using S-Log2 on the Sony A7s. Part One: Gamma and Exposure.
My suggested exposure levels for the Sony A7s. The “sweet spot” is from normal to +2 over.

If you don’t have an external waveform monitor then you do still have some good options. Sadly although the camera does have zebras, these are not terribly useful for S-log2 as the lowest the zebras can go is 70%.

Light Meter: You could use a conventional photography light meter. If you do choose to use a light meter I would recommend checking the calibration of the light meter against the camera first.

Mark 1 Eyeball: You could simply eyeball the exposure looking at the viewfinder or rear screen but this is tricky when the image is very flat.

In Camera Metering: The cameras built in metering system, like the majority of DSLR’s is calibrated for middle grey. By default the camera uses multi-point metering to measure the average brightness of several points across the scene to determine the scenes average brightness and from there set the correct base S-log2 exposure.

Auto Exposure:

When you are using S-Log2, auto exposure in most cases will be very close to the correct base exposure if you use the default Multi-Zone exposure metering. The camera will take an average exposure reading for the scene and automatically adjust the exposure to the Sony recommended 32% middle grey exposure level based on this average. In the P, A and S modes you can then use the exposure compensation dial to offset the exposure should you wish. My recommendation would be to add +1 or +2 stops via the dial. Then observe the histogram to ensure that you don’t have any significant over exposure. If you do then reduce the exposure compensation. Lots of peaks to the far right of the histogram is an indication of over exposure.

Manual Exposure And Internal Metering.

If you are exposing manually you will see a small M.M. indication at the bottom of the LCD display with a +/- number. In the eyepiece viewfinder this appears as a scale that runs from -5 to +5, in S-log2 only the -2 to +2 part of the scale is used. In both cases this is how far the camera thinks you are away from the optimum exposure. + meaning the camera is over exposed, – meaning under.

A7s-VF-MM Exposing and Using S-Log2 on the Sony A7s. Part One: Gamma and Exposure.
A7s Viewfinder indications in manual exposure mode showing both M.M. offset from metered exposure and histogram.

In the image above we can see the M.M. indication is +0.3, in the eyepiece you would see a small arrow one bar to the right of “0” , indicating the cameras multi zone metering thinks the shot is just a little over exposed, even though the shot has been carefully exposed using a grey card and external waveform monitor. This error is probably due to the large amount of white in the shot, white shirt, white card, test charts with a lot of brighter than grey shades.  In practice an error of 0.3 of a stop is not going to cause any real issues, so even if this was exposed by setting  the exposure so that you have “M.M. 0.0” the exposure would be accurate enough. But it shows that multi point exposure averaging is easily confused.

The scene above is a fairly normal scene, not excessively bright, not particularly dark. If shooting a snow scene for example the cameras multi point averaging would almost certainly result in an under exposed shot as the camera attempts to bring the bright snow in the scene down to the average middle grey level. If shooting a well lit face against a very dark background then the averaging might try to bring the background up and the shot may end up overexposed.

If you want really accurate exposure then you should put the cameras metering system into the spot metering mode where instead of taking an average of various points across the scene the camera will just measure the exposure at the very center of the image.

spot-metering-mode Exposing and Using S-Log2 on the Sony A7s. Part One: Gamma and Exposure.
A7s Spot Metering Mode.

You can then use a grey card to very accurately set the exposure. Simply place the circular shaped symbol at the center of the viewfinder display over a grey card and set the exposure so that M.M is 0.0 for the correct S-Log2 base exposure. To expose 1 stop over with a grey card, set M.M. +1.0 and two stops over M.M. +2.0 (not flashing, flashing indicates more than +2 stops).

MM-00-grey Exposing and Using S-Log2 on the Sony A7s. Part One: Gamma and Exposure.
Using Spot Metering to set exposure correctly for S-log2. MM 0.0.

One small issue with this is that the camera will only display a M.M. range of -2.0 to +2.0 stops. Provided you don’t want to go more than 2 stops over base then you will be fine with a grey card.

Using White Instead of Grey:

If you don’t have a grey card then you can use a 90% reflectivity white target. As white is 2 stops brighter than middle grey when S-Log2 is correctly exposed the 90% white should indicate M.M +2.0.

MM-plus2-white Exposing and Using S-Log2 on the Sony A7s. Part One: Gamma and Exposure.
Using spot metering to set the correct exposure for S-Log2. M.M should read M.M +2.0 for a 90% reflectivity white target.

Once you have established the correct exposure you can then open the iris by 1 or two stops to increase the exposure. Or halve the shutter speed to gain a one stop brighter exposure. Each time you halve the shutter speed your exposure becomes one stop brighter, so divide the shutter speed by 4 to gain a 2 stop increase in exposure. As always you should observe the histogram to check for any over exposure. White peaks at the far right of the histogram or disappearing completely off the right of the histogram is an indication of over-exposure. In this case reduce your exposure back down towards the base exposure level (M.M 0.0 with a a grey card).

Exposure Summary:

I recommend using an exposure between the “correct” base S-Log2 exposure level of middle grey at 32% and two stops over this. I would not recommend going more than 2 stops over over base.

In the P, A and S auto exposure modes, when using the default multi-zone metering the camera will set the base S-log2 exposure based on the average scene brightness. For most typical scenes this average should be very close to middle grey. This exposure can then be increased (brightened) by up to 2 stops using the exposure compensation dial.

In manual exposure the “M.M.” number displayed at the bottom of the viewfinder display is how far you are from the correct base S-log2 exposure. M.M. +2.0 indicates +2 stops over base. If using multi zone metering (the cameras default) this exposure will be based on the scenes average brightness.

If you set the metering to “Spot” you can use a grey card centred in the image to determine the correct base exposure and up to 2 stops of over exposure via the M.M. indication when shooting manually.

In Part 2:

In part two I will take a look at grading the S-log2 from the A7s and how to get the very best from the S-log2 images by using Look Up Tables (LUT’s).

PART 2 – How to handle the footage in post production is here.

NORTHERN LIGHTS Tours and workshops.

Don’t forget I run storm chasing and Northern Lights expeditions every year.  These are amazing expeditions by snowmobile up on to the Finnmarksvidda. We go ice fishing, dog sledding, exploring, cook a meal in a tent and enjoy traditional Norwegian saunas.

More information here.

sky-full-of-Aurora-1024x683 Exposing and Using S-Log2 on the Sony A7s. Part One: Gamma and Exposure.
Northern Lights over our cabins in Norway.

Major Update to my Cine-EI guide for the PMW-F55 and PMW-F5

I have just published a major update to my guide to Cine-EI on the PMW-F55 and F5. The guide now goes in to a lot more depth. I have tried to make it easy to understand but it is also quite technical, I have deliberately included the technical background stuff so that hopefully you will understand why Cine-EI and LUT’s work the way they do. I’ve added a whole new section on exposure methods for some of the different LUT’s as well as how to create your own LUT’s.

Please take a look if you use these cameras. Soon I will add a section on post production.

https://www.xdcam-user.com/2013/12/cine-ei-mode-when-recording-s-log23-and-raw-on-the-f5-and-f55/