Tag Archives: exposure

Beware the LC709 LUT double exposure offset.

The use o f the LC709 Type A LUT in Sony’s Cinealta cameras such as the PXW-FS7 or PMW-F55 is very common. This LUT is popular because it was designed to mimic the Arri cameras when in their Rec-709 mode. But before rushing out to use this LUT and any of the other LC709 series of LUT’s there are some things to consider.

The Arri cameras are rarely used in Rec-709 mode for anything other than quick turn around TV. You certainly wouldn’t normally record this for any feature or drama productions. It isn’t the “Arri Look” The Arri look normally comes as a result of shooting using Arri’s LogC and then grading that to get the look you want. The reason it exists is to provide a viewable image on set. It has more contrast than LogC and uses Rec 709 color primaries so the colors look right, but it isn’t Rec-709. It squeezes almost all of the cameras capture range into a something that can be viewed on a 709 monitor so it looks quite flat.

Because a very large dynamic range is being squeezed into a range suitable to be viewed on a regular, standard dynamic range monitor the white level is much reduced compared to regular Rec-709. In fact, white (such as a white piece of paper) should be exposed at around 70%. Skin tones should be exposed at around 55-60%.

If you are shooting S-Log on a Sony camera and using this LUT to monitor, if you were to expose using conventional levels, white at 85-90% skin tones at 65-70%, then you will be offsetting your exposure by around +1.5 stops. On it’s own this isn’t typically going to be a problem. In fact I often come across people that tell me that they always shoot at the cameras native EI using this LUT and get great, low noise pictures. When I dig a little deeper I often find that they are exposing white at 85% via the LC709 LUT. So in reality they are actually shooting with an exposure the equivalent of +1 to +1.5 stops over the base level.

Where you can really run into problems is when you have already added an exposure offset. Perhaps you are shooting on an FS7 where the native ISO is 2000 ISO and using an EI of 800. This is a little over a +1 stop exposure offset. Then if you use one of the LC709 LUT’s and expose the LUT so white is at 90% and skin tones at 70% you are adding another +1.5 stops to the exposure, so your total exposure offset is approaching 3 stops. This large an offset is rarely necessary and can be tricky to deal with in post. It’s also going to impact your highlight range.

So just be aware that different LUT’s require different white and grey levels and make sure you are exposing the LUT at it’s correct level so that you are not adding an additional offset to your desired exposure.

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Using LUT’s for exposure – choosing the right LUT.

If using a LUT to judge the exposure of a camera shooting log or raw it’s really important that you fully understand how that LUT works.

When a LUT is created it will expect a specific input range and convert that input range to a very specific output range. If you change the input range then the output will range will be different and it may not be correct. As an example a LUT designed and created for use with S-Log2 should not be used with S-Log3 material as the the higher middle grey level used by S-Log3 would mean that the mid range of the LUT’s output would be much brighter than it should be.

Another consideration comes when you start offsetting your exposure levels, perhaps to achieve a brighter log exposure so that after grading the footage will have less noise.

Lets look at a version of Sony’s 709(800) LUT designed to be used with S-Log3 for a moment. This LUT expects middle grey to come in at 41% and it will output middle grey at 43%. It will expect a white card to be at 61% and it will output that same shade of white at a little over 85%. Anything on the S-Log3 side brighter than 61% (white) is considered a highlight and the LUT will compress the highlight range (almost 4 stops) into the output range between 85% and 109% resulting in flat looking highlights. This is all perfectly fine if you expose at the levels suggested by Sony. But what happens if you do expose brighter and try to use the same LUT either in camera or in post production?

Well if you expose 1.5 stops brighter on the log side middle grey becomes around 54% and white becomes around 74%. Skin tones which sit half way between middle grey and white will be around 64% on the LUT’s input. That’s going to cause a problem! The LUT considers anything brighter than 61% on it’s input to be a highlight and it will compresses anything brighter than 61%. As a result on the output of your LUT your skin tones will not only be bright, but they will be compressed and flat looking. This makes them hard to grade. This is why if you are shooting a bit brighter it is much, much easier to grade your footage if your LUT’s have offsets to allow for this over exposure.

If the camera has an EI mode (like the FS7, F5, F55 etc) the EI mode offsets the LUT’s input so you don’t see this problem in camera but there are other problems you can encounter if you are not careful like unintentional over exposure when using the Sony LC709 series of LUTs.

Sony’s  709(800) LUT closely matches the gamma of most normal monitors and viewfinders, so 709(800) will deliver the correct contrast ie. contrast that matches the scene you are shooting plus it will give conventional TV brightness levels when viewed on standard monitors or viewfinders.

If you use any of the LC709 LUT’s you will have a miss-match between the LUT’s gamma and the monitors gamma so the images will show lower contrast and the levels will be lower than conventional TV levels when exposed correctly. LC709 stands for low contrast gamma with 709 color primaries, it is not 709 gamma!

Sony’s LC709 Type A LUT is very popular as it mimics the way an Arri Alexa might look. That’s fine but you also need to be aware that the correct exposure levels for this non-standard LC gamma are middle grey at around 41% and white at 70%.

An easy trap to fall into is to set the camera to a low EI to gain a brighter log exposure and then to use one of the LC709 LUT’s and try to eyeball the exposure. Because the LC709 LUT’s are darker and flatter it’s harder to eyeball the exposure and often people will expose them as you would regular 709. This then results in a double over exposure. Bright because of the intentional use of the lower EI but even brighter because the LUT has been exposed at or close to conventional 709 brightness. If you were to mistakenly expose the LC709TypeA LUT with skin tones at 70%, white at 90% etc then that will add almost 2 stops to the log exposure on top of any EI offset.

Above middle grey with 709(800) a 1 stop exposure change results in an a 20% change in brightness, with LC709TypeA the same exposure change only gives a just over 10% change, as a result over or under exposure is much less obvious and harder to measure or judge by eye with LC709. The cameras default zebra settings for example have a 10% window. So with LC709 you could easily be a whole stop out, while with 709(800) only half a stop.

Personally when shooting I don’t really care too much about how the image looks in terms of brightness and contrast. I’m more interested in using the built in LUT’s to ensure my exposure is where I want it to be. So for exposure assessment I prefer to use the LUT that is going to show the biggest change when my exposure is not where it should be. For the “look” I will feed a separate monitor and apply any stylised looks there. To understand how my highlights and shadows, above and below the LUT’s range are being captured I use the Hi/Low Key function.

If you are someone that creates your own LUT’s an important consideration is to ensure that if you are shooting test shots, then grading these test shots to produce a LUT it’s really, really important that the test shots are very accurately exposed.

You have 2 choices here. You can either expose at the levels recommended by Sony and then use EI to add any offsets or you can offset the exposure in camera and not use EI but instead rely on the offset that will end up in the LUT. What is never a good idea is to add an EI offset to a LUT that was also offset.

What does ISO mean with todays cameras?

Once upon a time the meaning of ISO was quite clear. It was a standardised sensitivity rating of the film stock you were using. If you wanted more sensitivity, you used film with a higher ISO rating. But today the meaning of ISO is less clear and we can’t swap our sensor out for more or less sensitive ones. So what does it mean?

ISO is short for International Standards Organisation. And they specify many, many different standards for many different things. For example ISO 3166 is for country codes, ISO 50001 is for energy management.

But in our world of film and TV there are two ISO standards that we have blended into one and we just call it “ISO”.

ISO 5800:2001 is the system used to determine the sensitivity of color negative film found by plotting the density of the film against exposure to light.

ISO 12232:2006 specifies the method for assigning and reporting ISO speed ratings, ISO speed latitude ratings, standard output sensitivity values, and recommended exposure index values, for digital still cameras.

Note a key difference: ISO 5800 is the measurement of the actual sensitivity to light of film.  ISO 12232 is a standardised way to report the speed rating, it is not a direct sensitivity measurement.

Within the digital camera ISO rating system there are 5 different standards that a camera manufacturer can use when obtaining the ISO rating of a camera. The most commonly used method is the Recommended Exposure Index (REI) method, which allows the manufacturer to specify a camera model’s EI or base ISO arbitrarily based on what the manufacturer believes produces a satisfactory image. So it’s not really a measure of the cameras sensitivity, but a rating that if used with a standard external calibrated light meter to set the exposure will give a satisfactory looking image. This is very different to a sensitivity measurement and variations in the opinion as to what is a satisfactory image will vary from person to person. So there is a lot of scope for movement as to how an electronic camera might be rated.

As you cannot change the sensor in a digital camera, you cannot change the cameras efficiency at converting light into electrons (which is largely determined by the materials used and the physical construction). So you cannot change the actual sensitivity of the camera to light. But we have all seen how the ISO number of most digital cameras can normally be increased (and sometimes lowered) from the base ISO number.

Raising and lowering the ISO in an electronic camera is normally done by adjusting the amplification of the signal coming from the sensor, typically referred to as “gain” in the camera. It’s not actually a physical change in the cameras sensitivity to light, it like turning up the volume on a radio to make the music louder. Dual ISO cameras that claim not to add gain when switching between ISO’s typically do this by adjusting the way the signal from the sensor is converted from an analog signal to a digital one. While it is true that this is different to a gain shift it does typically alter the noise levels as to make the picture brighter you need to sample the sensors output lower down and closer to the noise floor. Once again though it is not an actual sensitivity change, it does not alter the sensors sensitivity to light, you are just picking a different part of it’s output range.

Noise and Signal To Noise Ratio.

Most of the noise in the pictures we shoot comes from the sensor and the level of this noise coming from the sensor is largely unchanged no matter what you do (some dual ISO cameras use variations in the way the sensor signal is sampled to shift the noise floor up and down a bit). So the biggest influence on the signal to noise ratio is the amount of light you put on the sensor. More light = More signal. The noise remains the same but the signal is bigger so you get a better signal to noise ratio, up to the point where the sensor overloads.

But what about low light?

To obtain a brighter image when there the light levels are low and the picture coming from the sensor looks dark the signal coming from the sensor is boosted or amplified (gain is added). This amplification makes both the desirable signal bigger but also the noise bigger. If we make the desirable picture 2 times brighter we also make the noise 2 x bigger. As a result the picture will be more noisy and grainy than one where we had enough light to get the brightness we want.

The signal to noise ratio deteriorates because the added amplification means the recording will clip more readily. Something that is close to the recordings clip point may be sent above the clip point by adding gain, so the range you can record reduces while the noise gets bigger. However the optimum exposure is now achieved with less light so the equivalent ISO number is increased. If you were using a light meter you would increase the ISO setting on the light meter to get the correct exposure. But the camera isn’t more sensitive, it’s just that the optimum amount of light for the “best” or “correct” exposure is reduced due to the added amplification.

So with an electronic camera, ISO is a rating that will give you the correct brightness of recording for the amount of light and the amount of gain that you have. This is different to sensitivity. Obviously the two are related, but they are not quite the same thing.

Getting rid of noise:

To combat the inevitable noise increase as you add gain/amplification most modern cameras use electronic noise reduction which is applied more and more aggressively as you increase the gain. At low levels this goes largely un-noticed. But as you start to add more gain and thus and more noise reduction you will start to degrade the image. It may become softer, it may become smeary. You may start to see banding ghosting or other artefacts.

Often as you increase the gain you may only see a very small increase in noise as the noise reduction does a very good job of hiding the noise. But for every bit of noise thats reduced there will be another artefact replacing it.

Technically the signal to noise ratio is improved by the use of noise reduction, but this typically comes at a price and NR can be very problematic if you later want to grade or adjust the footage as often you won’t see the artefacts until after the corrections or adjustments have been made. So be very careful when adding gain. It’s never good to have extra gain.

Why are Sony’s ISO’s different between standard gammas and log?

With Sony’s log capable cameras (and most other manufacturers) when you switch between the standard gamma curves and log gamma there is a change in the cameras ISO rating. For example the FS7 is rated at 800 ISO in rec709 but rated at 2000 ISO in log. Why does this change occur and how does it effect the pictures you shoot?

As 709 etc has a limited DR (between around 6 and 10 stops depending on the knee settings) while the sensor itself has a 14 stop range, you only need to take a small part of the sensors full range to produce that smaller range 709 or hypergamma image. That gives the camera manufacturer some freedom to pick the sweetest part of the sensors range. his also gives some leeway as to where you place the base ISO.

I suspect Sony chose 800 ISO for the FS7 and F5 etc as that’s the sensors sweet spot, I certainly don’t think it was an accidental choice.

What is ISO on an electronic camera? ISO is the equivalent sensitivity rating. It isn’t a measure of the cameras actual sensitivity, it is the ISO rating you need to enter into a light meter if you were using an external light meter to get the correct exposure settings. It is the equivalent sensitivity. Remember we can’t change the sensor in these cameras so we can’t actually change the cameras real sensitivity, all we can do is use different amounts of gain or signal amplification to make the pictures brighter or darker.

When you go switch the camera to log you have no choice other than to take everything the sensor offers. It’s a 14 stop sensor and if you want to record 14 stops, then you have to take 100% of the sensors output. The camera manufacturer then chooses what they believe is the best exposure mid point point where they feel there is an acceptable compromise between noise, highlight and lowlight response. From that the manufacture will get an ISO equivalent exposure rating.

If you have an F5, FS7 or other Sony log camera, look at what happens when you switch from rec709 to S-Log2 but you keep your exposure constant.

Middle grey stays more or less where it is, the highlights come down. White will drop from 90% to around 73%. But the ISO rating given by the camera increases from 800ISO to 2000ISO. This increased ISO number implies that the sensor became more sensitive – This is not the case and a little missleading. If you set the camera up to display gain in dB and switch between rec709 (std gamma) and S-Log the camera stays at 0dB, this should be telling you that there is no change to the cameras gain, no change to it’s sensitivity. Yet the ISO rating changes – why?

The only reason the ISO number increases is to force us to underexpose the sensor by 1.3 stops (relative to standard gammas such as rec709 and almost every other gamma) so we can squeeze a bit more out of the highlights. If you were using an external light meter to set your exposure if you change the ISO setting on the light meter from 800 ISO to 2000 ISO  the light meter will tell you to close the aperture by 1.3 stops. So that’s what we do on the camera, we close the aperture down a bit to gain some extra highlight range.

But all this comes at the expense of the shadows and mid range. Because you are putting less light on the sensor if you use 2000 ISO as your base setting the shadows and mids are now not as good as they would be  in 709 or with the other standard gammas.

This is part of the reason why I recommend that you shoot with log between 1 and 2 stops brighter than the base levels given by Sony. If you shoot 1 stop brighter that is the equivalent to shooting at 1000 ISO and this is closer to the 800 ISO that Sony rate the camera at in standard gamma.  Shooting that bit brighter gives you a much better mid range that grades much better.

 

Why is exposing log brightly beneficial?

I have been asked whether you should still expose log a bit brighter than the recommended base levels on the Sony PXW-FS5 now that Sony have released new firmware that gives it a slightly lower base ISO. In this article I take a look at why it might be a good idea to expose log (with any camera) a bit brighter than perhaps the manufacturer recommends.

There are a couple of reasons to expose log nice and bright, not just noise. Exposing log brighter makes no difference to the dynamic range. That’s determined by the sensor and the gain point at which the sensor is working. You want the camera to be at it’s native sensitivity or 0dB gain to get that maximum dynamic range.

Exposing brighter or darker doesn’t change the dynamic range but it does move the mid point of the exposure range up and down.  Exposing brighter increases the under exposure range but decreases the over exposure range. Exposing darker decreases the under exposure range but increases the over exposure range.

Something that’s important when thinking about dynamic range and big dynamic ranges in particular is that dynamic range isn’t just about the highlights it’s also about the shadows, it isn’t just over exposure, it’s under exposure too, it’s RANGE.

So why is a little bit of extra light often beneficial? You might call it “over exposure” but that’s not a term I like to use as it implies “too much exposure”. I prefer to use “brighter exposure”.

It’s actually quite simple, it’s about putting a bit more light on to the sensor. Most sensors perform better when you put a little extra light on them. One thing you can be absolutely sure of – if you don’t put enough light on the sensor you won’t get the best pictures.

Slide01 Why is exposing log brightly beneficial?

Put more light on to the sensor and the shadows come up out of the sensors noise floor. So you will see further into the shadows. I’ve had people comment that “why would I ever want to use the shadows, they are always noisy and grainy”? But that’s the whole point – expose a bit brighter and the shadows will be much less noisy, they will come up out of the noise. Expose 1 stop brighter and you halve the shadow noise (for the same shadows at the previous exposure).  Shadows are are only ever noise ridden if you have under exposed them.

This is particularly relevant in controlled lighting. Say you light a scene for 9 stops. So you have 9 stops of dynamic range but a 14 stop sensor. Open up the aperture, put more light on the sensor, you get a better signal to noise ratio, less noisy shadows but no compromise of any type to the highlights because if the scene is 9 stops and you have 14 to play with, you can bring the exposure up by a couple of stops comfortably within the 14 stop capture range.

S-log-levels Why is exposing log brightly beneficial?
Chart showing S-Log2 and S-Log3 plotted against f-stops and code values. Note how little data there is for each of the darker stops, the best data is above middle grey. Note that current sensor only go to +6 stops ove middle grey so S-Log2 and S-Log record to different peak levels.

Look at the above diagram of Sony’s S-Log2 and S-Log3 curves. The vertical 0 line in the middle is middle grey. Note how above middle grey the log curves are more or less straight lines. That’s because above the nominal middle grey exposure level each stop is recorded with the same amount of data, this you get a straight line when you plot the curve against exposure stops. So that means that it makes very little difference where you expose the brighter parts of the image. Expose skin tones at stop + 1 or stop +3 and they will have a very similar amount of code values (I’m not considering the way dynamic range expands in the scene you shoot as you increase the light in the scene in this discussion). So it makes little difference whether you expose those skin tones at stop +1 or +3, after grading they will look the same.

Looking at the S-Log curve plots again note what happens below the “0” middle grey line. The curves roll off into the shadows. Each stop you go down has less data than the one before, roughly half as much. This mimics the way the light in a real scene behaves, but it also means there is less data for each stop. This is one of the key reasons why you never, ever want to be under exposed as if you are underexposed you mid range ends up in this roll off and will lack data making it not only noisy but also hard to grade as it will lack contrast and tonal information.

Open up by 1 additional stop and each of those darker stops is raised higher up the recording curve by one stop and every stop that was previously below middle grey doubles the amount of tonal values compared to before, so that’s 8 stops that will have 2x more data than before. This gives you a nice fat (lots of data) mid range that grades much better, not just because it has less noise but because you have a lot more data where you really need it – in the mid range.

Note: Skin tones can cover a wide exposure range, but typically the mid point is around 1 to 1.5 stops above middle grey. In a high contrast lighting situation skin tones will start just under middle grey and extend to about 2 stops over. If you accidentally under expose by 1 stop or perhaps don’t have enough light for the correct exposure you will seriously degrade the quality of your skin tones as half of your skin tones will be well below middle grey and in the data roll-off.

Now of course you do have to remember that if your scene does have a very large dynamic range opening up an extra stop might mean that some of the very brightest highlights might end up clipped. But I’d happily give up a couple of specular highlights for a richer more detailed mid range because when it comes to highlights – A: you can’t show them properly anyway because we don’t have 14 stop TV screens and B: because highlights are the least important part of our visual range.

A further consideration when we think about the highlights is that with log there is no highlight roll-off. Most conventional gamma curves incorporate a highlight roll-off to help increase the highlight range. These traditional highlight roll-offs reduce the contrast in the highlights as the levels are squeezed together and as a result the highlights contain very little tonal information. So even after grading they never look good, no matter what you do. But log has no highlight roll-off. So even the very brightest stop, the one right on the edge of clipping contains just as much tonal information as each of the other brighter than middle grey stops. As a result there is an amazingly large amount of detail than can be pulled out of these very bright stops, much more than you would ever be able to pull from most conventional gammas.

Compare log to standard gammas for a moment. Log has a shadow roll-off but no highlight roll-off. Most standard gammas have a strong highlight roll-off. Log is the opposite of standard gammas. With standard gammas, because of the highlight roll-off, we normally avoid over exposure because it doesn’t look good. With Log we need to avoid under exposure because of the shadow roll-off, it is the opposite to shooting with standard gammas.

As a result I strongly recommend you never, ever under expose log. I normally like to shoot log between 1 and 2 stops brighter than the manufacturers base recommendation.

Next week: Why is a Sony camera like the FS7,F5 800 ISO with standard gamma but 2000 ISO in log and how does that impact the image?

 

Beware Exposing To The Right With Log.

That may seem like quite a sensational headline – beware exposing to the right with log – but let me explain.

First of all, I’m not saying you can’t or shouldn’t expose to the right, all I am saying is beware – understand the implications.

First of all what is normally meant by exposing to the right? Well it’s a term that comes from the world of photography where you would use the cameras histogram to measure the exposure levels. Exposing to the right would normally mean setting the shutter speed and aperture so that the levels shown on the histogram are as far to the right as you can get them without going beyond the right side of the histogram. This would ensure a nice bright exposure with lots of light falling on the sensor, something that is normally highly desirable as you get a nice low noise picture once you have adjusted and processed it in your photo editing software.

You can expose to the right with a video camera too. However when shooting with Rec-709 or conventional gammas this can often result in nasty looking highlights thanks to the default knee settings, so it’s not normally a good idea for 709 and standard gammas.

With log or raw as there is no highlight roll off you can expose to the right and it should give you a nice bright exposure… or will it?????

The problem with exposing to the right is that you are exposing for the highlights in the scene. If shooting a low contrast or low dynamic range scene this isn’t going to cause any problems as exposing to the right will mean that everything in the scene is nice and bright.

But if shooting a high dynamic range scene, say an outdoor scene with bright clouds in the sky but large areas of shadow, the exposure will be optimised for the highlights. The mid range and shadows may end up too dark. On a sunny day if shooting a person with their back to the sun the sky could easily be 6 or 7 stops brighter than the skin tones. If you expose for the sky/highlights the skin tones will be 1 or 2 stops darker than the basic exposure level recommended for most  log curves.

(S-log2/3 has 14 stops. At the base exposure you have 6 stops above middle grey and 8 below. Skin tones are normally between 1 and 2 stops above middle grey. So if the sky/highlights are 6 stops above the skin tones, then exposing for the highlights will put the skin tones where middle grey should be, which is 1 stop under exposed and 2 stops below where I would normally like to see skin tones when shooting with log or raw).

The first thing a viewer will notice when they look at a scene with faces or people will be the skin tones. If these have been under exposed they will be grainy and less than ideal. The viewer will notice noise and grain and poor shadows long before they look at the brightest highlights. Shooting log and protecting the highlights or exposing to the right will often compromise the all important mid tones because you are exposing for the highlights, not the midrange. In addition exposing for highlights with a high dynamic range scene can often push the shadows down in level and they will end up noisy and grainy. The biggest issue with exposing to the right is that it’s extremely difficult to estimate how many stops there are between your mid tones and the highlights, so you never know quite where your mid tones are falling.

(Midtones – generally a white piece of paper or a 90% reflectivity white card would be considered to be the top end of the mid tones. Go down about 2.5 stops from white and you hit middle grey  (18% grey card). This range between middle grey and white is where skin tones, plants, most animals etc will be and it probably the most important part of most images).

An important consideration with log and raw is that there is no highlight roll off. Standard gammas (with the default knee found on almost every camera) , cinegammas, hypergammas etc all roll off the highlights. That is to say that as you approach the peak recording level the contrast is reduced as the highlights are squeezed together to try to extend the dynamic range. This reduction in contrast means that it is very difficult, if not impossible, to recover any nice, useable picture information out of anything close to the peak recording level. As a result with conventional gammas we tend to avoid over exposure at all costs as it looks nasty. This highlight roll off is one of the things that gives video the video look.

Log and raw don’t have this same kind of highlight roll off. The image gets brighter and brighter until it clips. With log the stop immediately below clipping contains just as much picture information as any other stop brighter than middle grey. With linear raw the stop just below clipping has more information than any other stop. As a result in post production there is a very large amount of data that can be pulled out of these highlights, even if they are a little clipped! So don’t worry about a few clipped highlights when shooting log. The other thing to remember is there is no TV or monitor that can show these highlights as they really are, so they will never look perfect anyway.

Another thing that happens when exposing to the right is that grading becomes harder than it needs to be. Because the separation between the mid tones and highlights will vary greatly depending on things like whether you are shooting into or away from the sun, when you expose to the right you mid tone brightness will be up and down all over the place. So in post production as well as adding the look that you want to your footage, you are also going to have to spend a lot of time matching the mid range exposure to balance skin tones etc from shot to shot.

Rather than exposing to the right what I recommend is exposing for the mid range. After all this is the important part of the image. To do this you need to use a diffuse reflective shade. The most commonly used shades are a 90% white card and/or an 18% reflectivity grey card – middle grey. Get the mid range right and in most cases the highlights will take care of themselves. Getting the mid range right might mean exposing the mid range  brighter than the recommended levels. But it’s the mid range we need to measure, not the highlights, this is the important part of the image.

90% white is an incredibly important level in the world of film and video. A typical piece of office paper reflects about 92-94% of the light falling on it. Office paper often uses brighteners and special chemicals to make it look bright and white. This white is the brightest diffuse surface you will likely ever see.  Anything brighter than this is normally going to be an actual source of light. The sky perhaps or a direct bounced reflection off a shiny, reflective surface such as the bodywork of a car. So anything brighter than 90% white would normally be considered to be a highlight and to us humans, highlights are visually less important than the mid range. This is why the knee on most video cameras kicks in at around 90%. Anything brighter than 90% is a highlight so the knee only effects highlights and leaves the all important mid range alone.

Middle grey is also very important because it’s a shade of grey that to most people looks to be half way between black and white. Skin tones fall roughly half way between middle grey and white. In addition if you average all the brightness levels within a typical scene the end result is typically very close to middle grey.  Light meters are calibrated to middle grey. The relationship between middle grey and white is fixed. White reflects 90%, middle grey 18%, no matter how bright the actual light source. So whether you are indoors, outside. Whether it’s sunny or overcast, white and middle grey will always be close to 2.5 stops apart. They are extremely useful fixed reference levels.

There are many ways to measure the brightness of a white or grey card. My preferred method is with a waveform display. But you could also use zebras (use a narrow zebra window if you can).  You can also use false colour. Unfortunately it’s very difficult to use a histogram to measure the brightness of a specific target. The histogram is a great measuring tool for photography, but less than ideal for video. If you can’t get a white/grey card out in front of the camera you could consider using a light meter. It’s also worth noting that skin tones sit just a little over half way between middle grey and white, so if you have no other reference you could simply place your skin tones a touch brighter than half way between the values you are targetting for middle grey and white.

Just to be clear: I do still recommend exposing Sony’s S-log2, S-log3 and raw between 1 and 2 stops brighter than the Sony base levels. But the key take-away is that it’s the mid range you need to measure and expose at this level. Exposing to the right using a histogram or waveform and just looking at the peaks and brightest parts of the image does not tell you what is happening in he mid range. Measure the mid range, not the peak brightness.

 

Guide to Cine EI – Still Current.

Just a reminder that my guide to shooting with Cine EI for the PMW-F5 and F55 cameras is still just as valid today as it was when I wrote it back in 2013. There have been a few tweaks to the cameras menu here and there, but the principles and basic operation have not changed.

So if you are new to Cine-EI please take a look at the guide. It takes you through how to shoot with Cine EI, which LUT’s to use and how to expose them.

Cine-EI Mode when recording S-Log2/3 and raw on the F5 and F55.

Why do I always shoot at 800 EI (FS7 and F5)?

This is a question that comes up time and time again. I’ve been using the F5 and FS7 for almost 5 years. What I’ve discovered in that time is that the one thing that people notice more than anything from these cameras is noise if you get your exposure wrong. In addition it’s much harder to grade a noisy image than a clean one.
Lets take a look at a few key things about how we expose and how the F5/FS7 works (note the same principle applies to most log based cameras, the FS5 also benefits from being exposed brighter than the suggested base settings).
What in the image is important? What will your audience notice first? Mid-range, shadows or highlights?
I would suggest that most audiences first look at the mid range – faces, skin tones, building walls, plants etc. Next they will notice noise and grain or perhaps poor, muddy or murky shadows. The last thing they will notice is a few very brightly highlights such as specular reflections that might be clipped.
The old notion of protecting the highlights comes from traditional gamma curves with a knee or highlight roll off where everything brighter than a piece of white paper (90% white) is compressed into a very small recording range. As a result when shooting with conventional gamma curves ALL of the brighter parts of the image are compromised to some degree, typically showing a lack of contrast and texture, often showing some weird monotone colors. Log is not like that, there is no highlight roll off, so those brighter than white highlights are not compromised in the same way.
 
In the standard gammas at 0dB the PXW-FS7, like the PMW-F5 is rated at 800 ISO. This gives a good balance between noise and sensitivity. Footage shoot at 0dB/800ISO with the standard gammas or Hypergammas generally looks nice and clean with no obvious noise problems. However when we switch to log the native ISO rating of the cameras becomes 2000 ISO, so to expose “correctly” we need to stop the aperture down by 1.3 stops. This means that compared to 709 and HG1 to HG4, the sensor is being under exposed by 1.3 stops. Less light on the sensor will mean more noise in the final image. 1.3 stops is the equivalent of 9dB. Imagine how Rec709 looks if it is under exposed by 1.3 stops or has to have +9dB of gain added in. Well – thats what log at 2000 ISO will look like.
 
However log has lots of spare headroom and no highlight compression. So we can choose to expose brighter than the base ISO because pushing that white piece of paper brighter in exposure does not cause it to become compressed.
If you open the aperture back up by 1.3 stops you get back to where you would be with 709 in terms of noise and grain. This would be “rating” the camera at 800 ISO or using 800 EI. Rating the camera at 800EI you still have 4.7 stops of over exposure range, so the only things that will be clipped will in most cases be specular reflections or extreme highlights. There is no TV or monitor in existence that can show these properly, so no matter what you do, they will never be true to life. So don’t worry if you have some clipped highlights, ignore them. Bringing your exposure down to protect these is going to compromise the mid range and they will never look great anyway.
 
You should also be extremely cautious about ever using an EI higher that 2000. The camera is not becoming more sensitive, people are often misslead by high EI’s into thinking somehow they are capturing more than they really are. If you were to shoot at 4000 EI you will end up with footage 15dB noisier than if you were shooting the same scene using 709 at 800 ISO. That’s a lot of extra noise and you won’t necessarily appreciate just how noisy the footage will be while shooting looking at a small monitor or viewfinder.
 
I’ve been shooting with the F5 and then the FS7 for almost 5 years and I’ve never found a situation where I going to an EI higher than 800 would have resulted in a better end result. At the same time I’ve seen a lot of 2000 EI footage where noise in the mid range has been an issue, one particular example springs to mind of a high end car shoot where 2000 EI was used but the gloss and shine of the car bodywork is spoilt because it’s noisy, especially the darker coloured cars.
 
Of course this is just my opinion, based on my own experience, others may differ and the best thing you can do is test for yourself.

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.