I had the pleasure of listening to Pablo Garcia Soriano the resident DiT/Colorist at the Sony Digital Motion Picture Center at Pinewood Studios last week talk about grading modern digital cinema video cameras during the WTS event .
The thrust of his talk was about exposure and how getting the exposure right during the shoot makes a huge difference in how much you can grade the footage in post. His main observation was that many people are under exposing the camera and this leads to excessive noise which makes the pictures hard to grade.
There isn’t really any real way to reduce the noise in a video camera because nothing you normally do can change the sensitivity of the sensor or the amount of noise it produces. Sure, noise reduction can mask noise, but it doesn’t really get rid of it and it often introduces other artefacts. So the only way to change the all important signal to noise ratio, if you can’t change the noise, is to change the signal.
In a video camera that means opening the aperture and letting in more light. More light means a bigger video signal and as the noise remains more or less constant that means a better signal to noise ratio.
If you are shooting log or raw then you do have a fair amount of leeway with your exposure. You can’t go completely crazy with log, but you can often over expose by a stop or two with no major issues. You know, I really don’t like using the term “over-expose” in these situations. But that’s what you might want to do, to let in up to 2 stops more light than you would normally.
In photography, photographers shooting raw have been using a technique called exposing to the right (ETTR) for a long time. The term comes from the use of a histogram to gauge exposure and then exposing so the the signal goes as far to the right on the histogram as possible (the right being the “bright” side of the scale). If you really wanted to have the best possible signal to noise ratio you could use this method for video too. But ETTR means setting your exposure based on your brightest highlights and as highlights will be different from shot to shot this means the mid range of you shot will go up and down in exposure depending on how bright the highlights are. This is a nightmare for the colorist as it’s the mid-tones and mid range that is the most important, this is what the viewer notices more than anything else. If these are all over the place the colorist has to work very hard to normalise the levels and it can lead to a lot of variability in the footage. So while ETTR might be the best way to get the very best signal to noise ratio (SNR), you still need to be consistent from shot to shot so really you need to expose for mid range consistency, but shift that mid range a little brighter to get a better SNR.
Pablo told his audience that just about any modern digital cinema camera will happily tolerate at least 3/4 of a stop of over exposure and he would always prefer footage with very slightly clipped highlights rather than deep shadows lost in the noise. He showed a lovely example of a dark red car that was “correctly” exposed. The deep red body panels of the car were full of noise and this made grading the shot really tough even though it had been exposed by the book.
When I shoot with my F5 or FS7 I always rate them a stop slower that the native ISO of 2000. So I set my EI to 1000 or even 800 and this gives me great results. With the F55 I rate that at 800 or even 640EI. The F65 at 400EI.
If you ever get offered a chance to see one of Pablo’s demos at the DMPCE go and have a listen. He’s very good.
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.
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.
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.
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.
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.
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%.
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.
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.
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:
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!
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.
A7s S-Log2 over exposed by one stop.A7s S-Log2 over exposed by 2 stops.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.
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.
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.
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 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.
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).
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.
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).
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.
With the UK set to see a couple of days of strong and severe thunderstorms I thought I would put together a very quick guide to shooting lightning with both stills cameras and video cameras. Your first issue will be finding somewhere dry to shoot from, you don’t want rain on your camera or lens. You also do need to consider safety. Lightning is dangerous, it can strike many miles from a thunderstorm. If you can hear thunder you are in the strike risk area, so do take care. One of the safest places to be in a thunderstorm is inside a car. If the car is struck the electricity will pass through the body of the car and not through the occupants, before jumping from the underside of the car to the ground. If you are shooting from a car stay inside the car, don’t sit with your feet out of the door or any part of you touching the ground. Don’t sit in the car while holding on to a camera on a tripod outside the car. Don’t stand under trees, they can explode when struck by lightning, don’t stand on the very top of a hill. Use your common sense.
Long exposure captures great nigh time lightning.
For either stills or video you’re really going to want to use a tripod to get the very best results. As you often get strong winds around thunderstorms you want a good stable tripod. If it is windy keep a close eye on the camera and tripod, you don’t want it blown over by a strong gust of wind.
A wide angle lens will increase your chances of getting a lightning bolt in your shot, but the wider the shot the less detail you will see in the lightning bolt. You can always crop in to a wide shot a bit if it’s too wide. I like to have something in the foreground to give some interest to the image, but try to avoid too many obstructions to the skyline as these will block your view of the lightning.
NIGHT FOCUS:
This is probably the easiest for still photos, but it has many challenges. One is focus as it’s hard to focus on a brief flash of lightning. You will need to use manual focus, autofocus will not work. Start by focussing on a very distant object, perhaps lights on the horizon, the moon, stars or any other VERY distant object, preferably a mile or more away. Then check and double check your focus. Lightning is very fine and if it’s out of focus it will ruin the shot. If you don’t have anything to focus on set the lens to infinity, the sideways “8” symbol is infinity and there will normally be a line to mark the point of infinity focus. Infinity is often NOT at the very end of the lenses focus travel so check for the proper infinity mark. By the way, take a torch/flashlight if your going out in the dark!
STILL PHOTO’s or DSLR AT NIGHT:
You will need to use a tripod. If you have a cable release or other electronic shutter release use it to trigger the camera to prevent shaking the camera as you will need to use a long exposure. As you will be using a long exposure you want to use a low ISO. I typically use 200ISO with an exposure of between 10 and 30 seconds depending on the frequency of the lightning and how bright the surrounding area is. If you are in a town or city with lots of street light you will probably need to use a shorter exposure, maybe 10 to 15 seconds. Out in the countryside you might be able to use 20 to 30 seconds. For the aperture you don’t want super shallow depth of field as this will show up any focus errors, so don’t use your lens wide open. I normally use somewhere around f4 to f8, so f5.6 is probably a good starting point. Take some test shots and check that you are not over exposed.
As a starting point try: 200ISO, f5.6, 10 second exposures, manual focus.
Once the camera is set, it simply a case of snapping away taking pictures until you get lucky and capture one in the frame. It takes a bit of luck and patience, but don’t give up too soon, just keep snapping away. You can just delete all the no good shots later.
Evening thunderstorm in Tucson, Arizona
DAY and NIGHT VIDEO:
If your camcorder has a CMOS sensor (as most do these days) you want to use the slowest shutter speed that you can get away with. If you can control the shutter manually turn it off or reduce it to 1/25 or 1/30. This will reduce the likelihood of you getting lightning bolts that only go half way down the screen, an effect know as “rolling shutter” or “flash band”. If shooting after dark, if you have a camera with full manual control then instead of shooting at the usual 24, 25 or 30 frames per second, consider shooting at half of this, perhaps at 12, 12.5 or 15 frames per second (S&Q motion, slow shutter etc), again with the shutter set to OFF. While this does mean that the motion in your final video will be sped up it almost guarantees that you won’t get any rolling shutter issues. You will need to have the camera on a tripod if doing this to prevent excessive image blur from movement of the camera. The slightly sped up video can also give the pleasing (but fake) impression that the lightning is more frequent than it really is making your shots more dramtic. If you don’t want this simply play the video back at half speed.
STILL PHOTOS DURING THE DAY:
This is really tough unless you have special equipment. You can’t use a long exposure as you would at night because the bright daytime light will wash out the lightning bolts.
Very often a lightning bolt is made up of several flashes in rapid succession. If you do have fast enough reactions and a fast enough camera, you can get the secondary flashes. You will need to use manual focus and manual exposure so there isn’t a delay while the camera thinks about focus and exposure which delays the release of the shutter. Use a tripod with a cable release or remote shutter and use a longish exposure, 1/30th or 1/15th as there can be up to 1/10th of a second delay between flashes and there could be multiple flashes, you don’t want too fast a shutter speed. Set your focus on a very distant object, use a low ISO, again I typically use 100 or 200 ISO. Shoot a couple of test images and set the aperture so that you have a very slightly underexposed shot, may -1EV to -1.5EV, the slightly darker overall image will help the bright lightning show up better. Then it’s just a case of pointing the camera at the storm on a tripod, with your finger on the trigger and try to hit that shutter release as soon as you see any lightning. I find it’s better to not look through the viewfinder, just look in the direction the camera is pointed. You may be lucky, maybe not, a lot will depend on the type of lightning in the storm and your reaction speed. A better way is to use a dedicated lightning trigger such as a Patchmaster: http://www.fotokonijnenberg.nl/patchmaster. This will trigger the camera electronically if it detects any lightning. It’s MUCH faster and can react much quicker than any human, but it still has some lag time so even a lightning trigger won’t capture every bolt.
A final daytime method is to use an adaptation of the night time DSLR method. If you add a strong ND filter a small aperture around f16 and use a low ISO you may be able to get an acceptable long exposure during daytime, perhaps a couple of seconds. Then set the camera to take photo’s continuously (so when you hold the shutter button down the camera will take one photo after another). By locking down a remote shutter release the camera will take a continuous stream of photos with only a very minimal gap between each picture taken. So you have a high likely hood of capturing any lightning bolts, but you will also end up with a lot of pictures that don’t have any lightning in them. You can either discard these empty frames or use all the frames to create a time-lapse video of the storm.
Have fun, stay safe.
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First of all. You can use either, LUT’s or Looks. But there is a quite marked difference in the way they behave, especially if you use EI gain.
At the native ISO there is little to choose between them. But just to confirm my earlier suspicions about the way the 3D LOOK’s behave I ran a quick test.
I found that when you lower the EI gain, below native, the output level of the LOOK lowers, so that depending on the EI, the clipping, peak level and middle grey values are different. For example on my PMW-F5 at 500 EI the LC709TypeA LUT has a peak output (clipping) level of just 90% while at 2000 ISO it’s 98%. This also means that middle grey of the LOOK will shift down slightly as you lower the EI. This means that for consistent exposure at different low EI’s you may need to offset your exposure very slightly. It also means that at Native EI if the waveform shows peak levels at 90% you are not overexposed or clipped, but at low EI’s 90% will mean clipped Slog, so beware of this peak level offset.
When you raise the EI of the LOOKS, the input clipping point of the Look profile changes. For each stop of EI you add the LOOK will clip one stop earlier than the underlying Slog. For example set the LC709TypeA LUT to 8000 ISO (on my PMW-F5) and the LOOK itself hard clips 2 stops before the actual SLog3 clips. So your LOOK will make it appear that your Slog is clipped up to 2 stops before it actually is and the dynamic range and contrast range of the LOOK varies depending on the EI, so again beware.
So, the Looks may give the impression that the Slog is clipped if you use a high ISO and will give the impression that you are not using your full available range at a low ISO. I suspect this is a limitation of 3D LUT tables which only work over a fixed 0 to1 input and output range.
What about the 1D LUT’s? Well the LUT’s don’t cover the full range of the Slog curves so you will never see all of your dynamic range at once. However I feel their behaviour at low and high EI’s is a little bit more intuitive than the level shifts and early clipping of the LOOKs.
The 1D LUT’s will always go to 109%. So there are no middle grey shifts for the LUT, no need to compensate at any ISO. In addition if you see any clipping below 109% then it means your SLog is clipping, for example if you set the camera to 500 ISO (on an F5), when you see the 709(800) LUT clipping at 105% it’s because the Slog is also clipping.
At High ISO’s you won’t see the top end of the SLog’s exposure range anyway because the LUT’s range is less than Slog’s range, but the LUT itself does not clip, instead highlights just go up above 109% and this is in my opinion more intuitive behaviour than the clipped LOOK’s that don’t ever quite reach 100% and clip at lower than 100% even when the Slog itself isn’t clipped.
At the end of the day use the ones that work best for you, just be aware of the limitations of both and that the LUT’s and LOOKs behave very differently. I suggest you test and try both before making any firm decisions.
Personally I prefer to use the 709(800) LUT for exposure as the restricted range matches that of most consumer TV’s etc so I feel this gives me a better idea of how the image may end up looking on a consumers TV. Also I find my Slog exposure more accurate as the LUT’s restricted range means you are more likely to expose within finer limits. In addition as noted above I fell the LUT’s behaviour is more predictable and intuitive at high and low EI’s than the LOOK’s.
In addition the higher contrast makes focus easier. I will often switch in and out of the LUT to look at how the Log is coping with any over exposure. This is my personal preference, but I do also use other LUT’s and Looks in particular the 709TypeA from time to time.
So we all like to dress our cameras up with all kinds of accessories. One of the most common being a Matte Box. So, what’s a matte box for? Well the obvious thing is to hold filters for creating an artistic look, for colour correction or light level reduction. But the other very important role is to block unwanted light. I’ll take a brief look at filters later in the article.
We all know that if you shoot into the sun or a bright light source you might get a lens flare in the shot. You know, those sometimes pretty rings of light that can look cool on a good day or ruin a shot on another. But the other thing you can get is lens flare. So whats the difference between “a lens flare” and “lens flare”.
Well, lens flare is when light bounces around inside the lens between the glass elements in an uncontrolled way, some of this unfocussed light making it’s way to the sensor where it spills and bleeds into darker parts of the image reducing contrast and raising the black level. Whenever you reduce the contrast in an image it will appear softer, so to get the sharpest and highest resolution images, we really want to keep as much unwanted light out of the lens as possible. In addition some cameras can suffer from other image artefacts when off-axis light finds it’s way to the edges of the sensor. So anything we can do to stop this happening is obviously a good thing.
Higher end cameras will often have an electronic flare adjustment that pulls down the cameras black level when the overall scene light level gets high. The idea is that this helps compensate for the almost inevitable flare that will occur in the lens when a lot of light enters the lens. This flare setting is normally adjusted on a lens by lens basis as different lenses will flare by different amounts. As lenses get older, very often vapour from the oils and materials used in the construction of the lens will coat the internal glass surfaces with a very fine haze that increases flare. This can make an older lens more prone to flare and is one reason why getting an older but expensive lenses professionally cleaned is often worth the expense. The other thing you can do is to make be sure to use a good matte box or lens shade to prevent excess light from entering the lens.
A flexible donut or “nun’s Knikers” on the rear of the Matte Box keeps out light from the rear of the Matte Box.
Don’t use a matte box that is excessively large. You want a Matte Box big enough to fit your lenses and hold the size of filters you need. It also needs to be wide enough to allow you to use the aspect ratios you want to shoot in, but no larger. If it’s too big, the shade/hood will be less effective. Make use of an adjustable top flag and side flags to keep out as much light as possible. Looking through the cameras viewfinder bring the flags in close to the lens until they start to creep into the edges of your shot, then back them off just a little bit.
A quick release catch on the Alphatron Matte Box allows you to quickly and easily change the donut or nun’s knickers.
Also make sure your rear donut or other light seal is doing it’s job and keeping out the light. A flexible bellows or “nun’s knickers” can be used to allow you to move the matte box forwards so that the lens sits deeper in the nice dark recess of the matte box. Light entering the Matte box from the rear will cause reflections off the back of any filters used, especially any ND filters or glimmer glass filters and this can easily spoil a shot.
A Matte Box can be attached to the lens directly via a clamp ring that clamps around the end of the lens or more commonly attached to rods or bars connected to the bottom of the camera. If you only ever use one lens then a lens clamp might work well for you, but if you swap and change lenses regularly then a rod or rail mount is often easier as a flexible donut will fit a multitude of lenses. The donut on the Alphatron Matte Box will fit a wide range of lenses and the neoprene insert can easily be exchanged or replaced simply by unscrewing the two halves of the donut holder. The neoprene is sandwiched between the two halves and just drops out once released.
Eyebrows on the Alphatron Matte box keeping the sun out of the lens.
Some Matte Boxes like the Alphatron one shown here have small extra “eyebrows”. These are like mini flags that can be adjusted to provide extra shade for the lens. In the picture you can see how the shadow from the top eyebrow is keeping stray and unwanted light from falling on the lens. This will help minimise flare and preserve contrast in the images. It’s a small thing but it can make a big difference. Eyebrows and flags also keep light out of the matte box itself and help prevent reflections between any filters that you might use and the lens itself.
The Alphatron Matte Box can be fitted with a quick release swing away adapter to make lens changes quick and easy.
If you’re using prime lenses then you will probably need to change lenses regularly. A great time saver is the use of a swing-away adapter. The Alphatron Matte Box that I use has an optional quick release swing away mount option. By twisting a single lever the Matte Box opens up and swings away from the lens. This gives you easy access to the lens for cleaning or for a quick lens change without having to remove the Matte Box. When shooting out on location this is a big deal as there’s never anywhere clean to put your Matte Box when you want to do a lens swap.
The Alphatron Matte Box has one fixed filter tray and one rotating tray.
Matte boxes can have both fixed and rotating filter trays or a combination of the two. Fixed trays are fine for ND filters and most diffusion filters. For graduated filters a rotating tray is preferable and for polarising filters a rotating tray is essential. The Alphatron Matte Box here has one fixed tray and one rotating tray. So I can use the fixed tray for any ND filters and then the rotating tray for grads or polarisers. I very nice feature of the Alphatron is a little recess in the very front of the sun shade and a little locking tab that allows you to put a safety glass in place in front of any filters to protect you filters and lens. This is very handy especially if your shooting something that could possibly splash on your expensive filters and damage the coatings.
So what filters should you get for your nice new Matte Box? First of all do be prepared to spend a little bit of money to get good quality filters. Filters can be plastic, resin or glass. Optical grade plastics and resins can make very good filters, but they tend to be prone to collecting dust through static electricity and they scratch easily. In addition if left in a hot car they can distort and warp. But, plastic and resin filters are light weight and normally a lot cheaper than the glass equivalent. Better quality filters will have anti-reflective coatings. A good quick test of the quality of any filter is to use a long focal length lens or zoomed-in zoom lens to check for distortions or focus issues introduced by the filter. You might not notice this at wide angles or zoomed out. So do check at longer focal lengths.
Good brands include Tiffen, Formatt and Schneider. These won’t be the cheapest on the market, but the quality is consistently good. Filters come in different sizes, the most common is the 4×4 or 4″ by 4″. For longer focal lengths these are fine, but if you want to shoot at wider angles you may find that 4×4’s are not wide enough. The next size up is the 4″x 5″ but the next commonly used size is the 4″ x 5.65″ which is close to the old 4:3 TV aspect ratio. The extra width really helps when shooting wider shots in 16:9.
My most commonly used filters are ND filters. These help manage light when it’s too bright allowing you to use a smaller aperture to gain a shallower depth of field. If your using a CMOS camera you should use IR ND filters that cut not only the visible light but also infra red light that most CMOS cameras are sensitive to.
Next is a polarising filter. A circular polariser is great for reducing or controlling reflections from windows, cars etc, it’s also good for enhancing the contrast in clouds and the sky making the sky a richer, deeper blue. When using a polariser it needs to go in a rotating tray so you can turn it when composing your shot to alter the polarising effect.
Graduated ND filters are also useful to help deal with excessively bright sky. The top of the filter is a ND filter or coloured filter and the bottom is normally clear. By sliding the filter up and down within the matte box you can alter level where the brightness reduction takes place. A tobacco or orange coloured graduated filter can be used to create or enhance a sunset type look. Just watch for the graduation crossing through foreground objects in the scene which can give the game away and look odd.
One way to reduce the noise in a video camera image is to reduce the cameras gain. One way to increase the brightness of the image is to add gain.
We all know that increasing the gain to lets say +6db will increase noise and generally the reverse holds true when you reduce the gain, the noise typically reduces and this may be helpful if you are going to do a lot of effects work, or just want a clean image.
However in most cases adding or removing gain reduces the cameras dynamic range as it will artificially clip or limit your low key or high key parts of the image. The maximum illumination level that a camera can capture is limited by the sensor or the gamma curves that the camera has. The black level or darkest part of the image is the point where the actual image signal compared to the sensor noise level is high enough to allow you to see some actual picture information (also known as noise floor). So the dynamic range of the camera is normally the range between the sensors noise floor and recording or sensor clipping point.
To maximise the cameras dynamic range the designers will have carefully set the nominal zero db gain point (native ISO) so that the noise floor is at or very close to black and the peak recording level is reached at the point where the sensor itself starts to clip.
The gain of the camera controls the video output and recording level, relative to the sensors signal level. If you use -3db gain you attenuate (reduce) the relative output signal. The highlight handling doesn’t change (governed by the sensor clipping or gamma curve mapping) but your entire image output level gets shifted down in brightness and as a result you will clip off or loose some of your shadow and dark information, so your overall dynamic range is also reduced as you can’t “see” so far into the shadows. Dynamic range is not just highlight handling, it is the entire range from dark to light. 3db is half a stop (6db = 1 stop) so -3db gain reduces the dynamic range by half a stop, reducing the cameras underexposure range without (in most cases) any change to the over exposure range, so overall the total dynamic range is reduced.
When you add gain the reverse happens. Generally how far the sensor can see into the shadows is limited by the sensors noise floor. Add 6db of gain and you will make the darkest parts of the image brighter by 6db, but you will also raise the noise level by the same amount. So while you do end up with brighter shadow details you can’t actually see any more picture information because the noise level has increased by the same amount. At the top end as the brightest sensor output is mapped to the maximum recording level at 0db, when you add gain this pushes the recording level beyond what can be recorded, so you loose 6db off the top end of your recordings because the recordings and output clips 6db earlier. So positive gain maintains the same shadow range but reduces the highlight recording range by 6db.
However you use it gain tends to reduce your dynamic range. Adding gain to cope with poor lighting tends to be the lesser of the two evils as generally if your struggling for light then overexposure and blown out highlights is often the last of your worries.
Negative gain is sometimes used in camera to try to reduce noise, but the reality is that you are loosing dynamic range. Really a better solution would be to expose just a tiny bit brighter and then bring your levels down a bit in post production.
This question comes up a lot, which gamma curve will give me the best results on a low key (dark) shoot. A common example would be “Should I use Gamma 2 which is brighter or should I use Gamma 5 which is darker but with a bit of gain”? or “Which is better to use in low light, normal gamma or log gamma”?
Log gamma (SLog, LogC etc) is great for capturing scenes with a large dynamic range, but it comes at the price of compressed highlights and overall less data per stop of exposure. Standard gammas have a lower dynamic range, but as a result the amount of data you are recording per stop of exposure is greater. It’s also vital to remember that if you do shoot in the dark with log you still need to expose correctly keeping middle grey and white at the correct levels in order that you keep mid tones and skin tones out of the heavily compressed part of the curve for the best results, so your images may look very dark and very flat. This can make focussing a big challenge (Top Tip – Use a large waveform monitor to check focus, as you go in and out of focus you should see the fine details in the waveform increase in amplitude as you become more focussed).
If you don’t need 13 or 14 stops of range, then use another gamma, don’t use log. Again with any of the extended range gammas like Cinegamma or Hypergamma watch where you place your skin tones, your exposure levels with these types of gamma curve are normally a little lower than with standard gammas (typically -0.5 to -1 stop).
A basic principle to understand with any video camera is that the actual sensitivity of the camera is purely a function of the sensor. Gain or raising the ISO is simply amplifying the signal off the sensor and results in more noise as the noise gets amplified too. Just like a music amplifier where you can make the music louder by turning up the volume (adding gain) you can make your pictures louder (brighter) by adding gain or raising the ISO. Listen to the music amplifier at high volume levels and what do you hear in the background? More hiss and noise, it’s the same with your video camera, more gain = more noise.
Gamma is also a form of gain providing different amounts of gain for different sensor output (brightness) levels to achieve specific recording and camera output levels, but it’s still gain (it can sometime be negative gain). The nearest you have to a gamma control on an audio amplifier would be an equaliser or bass and treble tone controls. Turn up the treble control on an audio amp and again you will here more hiss and noise, it’s nothing more than a selective gain control that shapes the sound you hear. Video camera gamma is very similar, it shapes the tonal range of the images you see.
So – and this is the bottom line: With any given gamma, combined with any given ISO or gain level, to achieve any particular brightness of output, for the same input level the total system gain (gain + gamma) is the same for that particular point in the scenes brightness range. So the noise level for any given brightness of output will be almost exactly the same whether you mix ISO “A” with Gamma “B” or ISO “B” with Gamma “A” because the combined gain of A + B is the same as B + A. At the end of the day the real sensitivity is governed by the sensor and anything else that makes the image brighter is gain, either regular “gain” or gamma gain.
So, choose a gamma curve that can deal with the dynamic range that you need and no more, don’t waste recording data on unused dynamic range. That way when you grade or do any post production noise reduction you have more data per stop to work from and that will help with the final image. If you do use log or a high range gamma (Hypergamma, Cinegamma etc) watch your exposure levels, the same rules apply in the dark as in daylight, you don’t want to over expose any skin tones as they won’t grade nicely.
When an engineer designs a gamma curve for a camera he/she will be looking to achieve certain things. With Sony’s Hypergammas and Cinegammas one of the key aims is to capture a greater dynamic range than is possible with normal gamma curves as well as providing a pleasing highlight roll off that looks less electronic and more natural or film like.
Recording a greater dynamic range into the same sized bucket.
To achieve these things though, sometimes compromises have to be made. The problem being that our recording “bucket” where we store our picture information is the same size whether we are using a standard gamma or advanced gamma curve. If you want to squeeze more range into that same sized bucket then you need to use some form of compression. Compression almost always requires that you throw away some of your picture information and Hypergamma’s and Cinegamma’a are no different. To get the extra dynamic range, the highlights are compressed.
Compression point with Hypergamma/Cinegamma.
To get a greater dynamic range than normally provided by standard gammas the compression has to be more aggressive and start earlier. The earlier (less bright) point at which the highlight compression starts means you really need to watch your exposure. It’s ironic, but although you have a greater dynamic range i.e. the range between the darkest shadows and the brightest highlights that the camera can record is greater, your exposure latitude is actually smaller, getting your exposure just right with hypergamma’s and cinegamma’s is very important, especially with faces and skin tones. If you overexpose a face when using these advanced gammas (and S-log and S-log2 are the same) then you start to place those all important skin tone in the compressed part of the gamma curve. It might not be obvious in your footage, it might look OK. But it won’t look as good as it should and it might be hard to grade. It’s often not until you compare a correctly exposed sot with a slightly over shot that you see how the skin tones are becoming flattened out by the gamma compression.
But what exactly is the correct exposure level? Well I have always exposed Hypergammas and Cinegammas about a half to 1 stop under where I would expose with a conventional gamma curve. So if faces are sitting around 70% with a standard gamma, then with HG/CG I expose that same face at 60%. This has worked well for me although sometimes the footage might need a slight brightness or contrast tweak in post the get the very best results. On the Sony F5 and F55 cameras Sony present some extra information about the gamma curves. Hypergamma 3 is described as HG3 3259G40 and Hypergamma 4 is HG4 4609G33. What do these numbers mean? lets look at HG3 3259G40
The first 3 numbers 325 is the dynamic range in percent compared to a standard gamma curve, so in this case we have 325% more dynamic range, roughly 2.5 stops more dynamic range. The 4th number which is either a 0 or a 9 is the maximum recording level, 0 being 100% and 9 being 109%. By the way, 109% is fine for digital broadcasting and equates to bit 255 in an 8 bit codec. 100% may be necessary for some analog broadcasters. Finally the last bit, G40 is where middle grey is supposed to sit. With a standard gamma, if you point the camera at a grey card and expose correctly middle grey will be around 45%. So as you can see these Hypergammas are designed to be exposed a little darker. Why? Simple, to keep skin tones away from the compressed part of the curve.
Here are the numbers for the 4 primary Sony Hypergammas:
The PMW-F5 and F55 are fantastic cameras. If you have the AXS-R5 raw recorder the dynamic range is amazing. In addition because there is no gamma applied to the raw material you can be very free with where you set middle grey. Really the key to getting good raw is simply not to over expose the highlights. Provided nothing is clipped, it should grade well. One issue though is that there is no way to show 14 stops of dynamic range in a pleasing way with current display or viewfinder technologies and at the moment the only exposure tool you have built in to the F5/F55 cameras are zebras.
My experience over many shoots with the camera is that if you set zebras to 100% and don’t use a LUT (so your monitoring using S-Log2) and expose so that your just starting to see zebra 2 (100%) on your highlights, you will in most cases have 2 stops or more of overexposure headroom in the raw material. Thats fine and quite useable, but shoot like this and the viewfinder images will look very flat and in most cases over exposed. The problem is that S-Log 2’s designed white point is only 59% and middle grey is 32%. If your exposing so your highlights are at 100%, then white is likely to be much higher than than the designed level, which also means middle grey and your entire mid range will be excessively high. This then pushes those mids into the more compressed part of the curve, squashing them all together and making the scene look extremely flat. This also has an impact on the ability to focus correctly as best focus is less obvious with a low contrast image. As a result of the over exposed look it’s often tempting to stop down a little, but this is then wasting a lot of available raw data.
So, what can you do? Well you can add a LUT. The F5 and F55 have 3 LUTS available. The LUTS are based either on REC709 (P1) or Hypergamma (P2 and P3). These will add more contrast to the VF image, but they show considerably less dynamic range than S-Log2. My experience with using these LUT’s is that on every shoot I have done so far, most of my raw material has typically had at least 3 stops of un-used headroom. Now I could simply overexpose a little to make better use of that headroom, but I hate looking into the viewfinder and seeing an overexposed image.
Why is it so important to use that extra range? It’s important because if you record at a higher level the signal to noise ratio is better and after grading you will have less noise in the finished production.
Firmware release 1.13 added a new feature to the F5 and F55, EI Gain. EI or Exposure Index gain allows you to change the ISO of the LUT output. It has NO effect on the raw recordings, it ONLY affects the Look Up Tables. So if you have the LUT’s turned on, you can now reduce the gain on the Viewfinder, HDSDI outputs as well as the SxS recordings (see this post for more on the EI gain). By using EI gain and an ISO lower than the cameras native ISO I can reduce the brightness of the view in the viewfinder. In addition the zebras measure the signal AFTER the application of the LUT or EI gain. So if you expose using a LUT and zebra 2 just showing on your highlights and then turn on the EI gain and set it to 800 on an F5 (native 2000ISO) or 640 on an F55 (native 1250ISO) and adjust your exposure so that zebra 2 is once agin just showing you will be opening your aperture by 1.5 (F5) or 1 (F55) stop. As a result the raw recordings will be 1.5/1 stop brighter.
In order to establish for my own benefit which was the best EI gain setting to use I spent a morning trying different settings. What I wanted to find was a reliable way to expose at a good high level to minimise noise but still have a little headroom in reserve. I wanted to use a LUT so that I have a nice high contrast image to help with focus. I chose to concentrate on the P3 LUT as this uses hypergamma with a grey point at 40% so the mid range should not look underexposed and contrast would be quite normal looking.
When using EI ISO 800 and exposing the clouds in the scene so that zebras were just showing on the very brightest parts of the clouds the image below is what the scene looked like when viewed both in the viewfinder and when opened up in Resolve. Also below is the same frame from the raw footage both before and after grading. You can click on any of the images to see a larger view.
P3 LUT, XDCAM recording, 800 EI ISO (PMW-F5).Raw footage, EI 800 ISO pre-grade.Raw, 800 ISO after grade. NO clipped highlights.
As you can see using LUT P3 and 800 EI ISO (PMW-F5) and zebra 2 just showing on the brightest parts of the clouds my raw footage is recorded at a level roughly 1.5 stops brighter than it would have been if I had not used EI gain. But even at this level there is no clipping anywhere in the scene, so I still have some extra head room. So what happens if I expose one more stop brighter?
The XDCAM recording, LUT P3, 800 EI, +1 stop, zebras showing over almost all clouds.Raw clip at +1 stop prior to grade.Raw at +1 stop after grade, no sign of any clipping.
So, as you can see above even with zebras over all of the brighter clouds and the exposure at +1 stop over where the zebras were just appearing on the brightest parts of the clouds there was no clipping. So I still have some headroom left, so I went 1 stop brighter again. The image in the viewfinder is now seriously over exposed.
The XDCAM recording at +2 stops, the sky and clouds look very overexposed.Raw clip, pre grading (LUT P3, EI 800). Looking scarily over exposed.After the grade the raw is looking much better, but there is a bit of clipping on the very brightest clouds.
The lower of the 3 images above is very telling. Now there is some clipping, you can see it on the waveform. It’s only on the very brightest clouds, but I have no reached the limit of my exposure headroom.
Based on these tests I feel very comfortable exposing my F5 in raw by using LUT P3 with EI gain at 800 and having zebra 2 starting to appear on my highlights. That would result in about 1.5 stops of headroom. If you are shooting a flat scene you could even go to 640 ISO which would give you one safe stop over the first appearance of zebra 2. On the F55 this would equate to using EI 640 with LUT P3 and having a little over 1.5 stops of headroom over the onset of zebras or EI 400 giving about 1 stop of headroom.
My recommendation having carried out these tests would be to make use of the lower EI gain settings to brighten your recorded image. This will result in cleaner, lower noise footage and also allow you to “see” a little deeper into the shadows in the grade. How low you go will depend on how much headroom you want, but even if you use 640 on the F5 or 400 on the F55 you should still have enough headroom above the onset of zebra 2 to stay out of clipping.
This one keeps coming around again and again and it’s not well understood by many.
When the standards for SDI and connecting devices via SDI were originally set down everyone was using interlace. The only real exception was people producing movies and films in 24p. In the 1990’s there became a need to transfer film scans to digital tape and to connect monitors to film scanners. The led to the adoption of a method of splitting a progressive frame into two halves by splitting out the odd and the even numbered lines and then passing these two halves of the progressive frame within a conventional interlaced signal.
In effect the odd numbered lines from the progressive frame were sent in what would be the upper field of an interlace stream and then the even numbered lines in what would be the lower field. So in effect the progressive frame gets split into two fields, a just like an interlaced video stream, but as the original source is progressive there is no time difference (temporal difference) between when the odd and even are were captured, so despite the split, what is passed down the SDI cable is still a progressive frame. This is PsF (Progressive Segmented Frame).
This system has the added benefit that even if the monitor at the end of the SDI chain is interlace only, it will still display the progressive material more or less correctly.
But here’s the catch. Because the progressive frame, split into odd and even lines and then stuffed into an interlace signal looks so much like an interlace signal, many devices attached to the PsF source cannot distinguish PsF from real interlace. So, more often than not the recorder/monitor/edit system will report that what it is receiving is interlace, even if it is progressive PsF. In most cases this doesn’t cause any problems as what’s contained within the stream does not have any temporal difference between the odd and even lines. The only time it can cause problems is when you apply slow motion effects, scaling effects or standards conversion processes to the footage as fields/lines from adjacent frames may get interleaved in the wrong order. Cases of this kind of thing are however quite rare and unusual.
Some external recorders offer you the option to force them to mark any files recorded as PsF instead of interlace. If you are sure that what you are sending to the recorder is progressive, then this is a good idea. However you do need to be careful because what will screw you up is marking real interlace footage as PsF by mistake. If you do this the interlaced frames will be treated as progressive. If there is any motion in the frame then the two true interlace fields will contain objects in different positions, they will have temporal differences. Combine those two temporally different fields together into a progressive frame and you will see an artefact that looks like a comb has been run through the frame horizontally, it’s not pretty and it can be hard to fix.
So, if you are shooting progressive and your external recorder or other device say’s it’s seeing interlace from your HDSDI, don’t panic. This is quite normal and you can continue to record with it.
If you are importing footage that is indicated as being interlace, but you know it’s progressive PsF into most edit packages you can normally select the clips and “interpret footage” or similar to change the clip header files to progressive instead of interlace and again all will be fine.
UPDATE: Since first writing this the use of a true 24/25/30p progressive output has become far more common. PsF still remains a perfectly valid ITU/SMPTE standard for Progressive, but not every monitor supports it. Early implementations of 24/25/30p over SDI were often created using non standard methods and as a result there are many cameras, monitors and recorders that support a 24/25/30p input or output, but may not be compatible with devices from other manufacturers. The situation is improving now, but issues remain due to the multitude of different standards and non standard devices. If you are having compatibility issues sometimes going up to 50p/60p will resolve it as the standards for 50/60p are much better defined. Or perhaps you may need to use a device such as a Decimator between the output and input to convert or standardise the signal.
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