Raw is the unprocessed (or minimally processed) data direct from the sensor. It is just the brightness value for each of the pixels, it is not a color image, but we know which color filter is above each pixel, so we are able to work out the color later. In the computer you take that raw data and convert it into a conventional color video signal defining the gamma curve and colorspace in the computer. This gives you the freedom to choose the gamma and colorspace after the shoot and retains as much of the original sensor information as possible.Of course the captured dynamic and color range is determined by the capabilities of the sensor and we can’t magically get more than the sensor can “see”. The quality of the final image is also dependant on the quality of the debayer process in the computer, but as you have the raw data you can always go back and re-encode the footage with a better quality encoder at a later date. Raw can be compressed or uncompressed. Sony’s 12 bit FS-raw when recorded on an Odyssey or Atomos recorder is normally uncompressed so there are no additional artefacts from compression, but the files are large. The 16 bit raw from a Sony F5 or F55 when recorded on an R5 or R7 is made about 3x smaller through a proprietary algorithm.
ProRes is a conventional compressed color video format. So a ProRes file will already have a pre-determined gamma curve and color space, this is set in the camera through a picture profile, scene file or other similar settings at the time of shooting. The quality of the ProRes file is dependant on the quality of the encoder in the camera or recorder at the time of recording, so there is no way to go back and improve on this or change the gamma/colorspace later. In addition ProRes, like most commonly used codecs is a lossy compressed format, so some (minimal) picture information may be lost in the encoding process and artefacts (again minimal) are added to the image. These cannot easily be removed later, however they should not normally present any serious problems.
It’s important to understand that there are many different types of raw and many different types of ProRes and not all are equal. The FS-raw from the FS5/FS7 is 12 bit linear and 12 bit’s are not really enough for the best possible quality from a 14 stop camera (there are not enough code values so floating point math and/or data rounding has to take place and this effects the shadows and low key areas of the image). You really need 16 bit data for 14 stops of dynamic range with linear raw, so if you are really serious about raw you may want to consider a Sony F5 or F55. ProRes is a pretty decent codec, especially if you use ProResHQ and 10 bit log approaches the quality of 12 bit linear raw but without the huge file sizes. Incidentally there is very little to be gained by going to ProRes 444 when recording the 12 bit raw from an FS5/FS7, you’ll just have bigger files and less record time.
Taking the 12 bit raw from an FS5 and converting it to ProRes in an external recorder has potential problems of it’s own. The quality of the final file will be dependant on the quality of the debayer and encoding process in the recorder, so there may be differences in the end result from different recorders. In addition you have to add a gamma curve at this point so you must be careful to choose the correct gamma curve to minimise concatenation where you add the imperfections of 12 bit linear to the imperfections of the 10 bit encoded file (S-Log2 appears to be the best fit to Sony’s 12 bit linear raw).
Despite the limitations of 12 bit linear, it is normally a noticeable improvement over the FS5’s 8 bit internal UHD recordings, but less of a step up from the 10 bit XAVC that an FS7 can record internally. What it won’t do is allow you to capture anything extra. It won’t improve the dynamic range, won’t give you more color and won’t enhance the low light performance (if anything there will be a slight increase in shadow noise and it may be slightly inferior in under exposed shots). You will have the same dynamic and color range, but recorded with more “bits” (code values to be precise). Linear raw excels at capturing highlight information and what you will find is that compared to log there will be more textures in highlights and brighter parts of your captured scenes. This will become more and more important as HDR screens are better able to show highlights correctly. Current standard dynamic range displays don’t show highlights well, so often the extra highlight data in raw is of little benefit over log. But that’s going to change in the next few years so linear recording with it’s extra highlight information will become more and more important.
Unfortunately every now and again a new term or buzzword comes along that gets taken as a holy grail term. Two that come to mind right now are log and raw. Neither log, nor raw, are magic bullet solutions that guarantee the best performance. Used incorrectly or inappropriately both can result in inferior results. In addition there are many flavours of log and raw each with very different performance ranges.
A particular point in case is the 12 bit raw available from several of Sony’s mid range large sensor cameras, the FS700, FS7 and FS5.
Raw can be either log or linear. This particular flavour of raw is encoded using linear data. If it is linear then each successively brighter stop of exposure should be recorded with twice as many code values or shades as the previous stop. This accurately replicates the change in the light in the scene you are shooting. If you make the scene twice as bright, you need to record it with twice as much data. Every time you go up a stop in exposure you are doubling the light in the scene. 12 bit linear raw is actually very rare, especially from a camera with a high dynamic range. To my knowledge, Sony are the only company that offer 14 stops of dynamic range using 12 bit linear data.
There’s actually a very good reason for this: Strictly speaking, it’s impossible! Here’s why: For each stop we go up in exposure we need twice as many code values. With 12 bit data there are a maximum of 4096 code values, this is not enough to record 14 stops.
If stop 1 uses 1 code value, stop 2 will use 2, stop 3 will use 4, stop 4 will use 8 and so on.
As you can see from the above if we only have 12 bit data and as a result 4096 code values to play with, we can only record an absolute maximum of 12 stops of dynamic range using linear data. To get even 12 stops we must record the first couple of stops with an extremely small amount of tonal information. This is why most 14 stop raw cameras use 16 bit data for linear or use log encoded raw data for 12 bit, where each stop above middle grey (around stop +8) is recorded with the same amount of data.
So how are Sony doing it on the FS5, FS7 etc? I suspect (I’m pretty damn certain in fact) that Sony use something called floating point math. In essence what they do is take the linear data coming off the sensor and round the values recorded to the nearest 4 or 8. So, stop +14 is now only recorded with 2,048 values, stop +13 with 512 values etc. This is fine for the brighter stops where there are hundreds or even thousands of values, it has no significant impact on the brighter parts of the final image. But in the darker parts of the image it does have an impact as for example stop +5 which starts off with 16 values ends up only being recorded with 4 values and each stop below this only has 1 or two discreet levels. This results in blocky and often noisy looking shadow areas – a common complaint with 12 bit linear raw. I don’t know for a fact that this is what they are doing. But if you look at what they need to do, the options available and you look at the end results for 12 bit raw, this certainly appears to be the case.
Meanwhile a camera like the FS7 which can record 10 bit log will retain the full data range in the shadows because if you use log encoding, the brighter stops are each recorded with the same amount of data. With S-Log2 and 10 bit XAVC-I the FS7 uses approx 650 code values to record the 6 brightest stops in it’s capture range reserving approx 250 code values for the 8 darkest stops. Compare this to the linear example above and in fact what you will see is that 10 bit S-Log2 has as much data as you would expect to find in a straight 16 bit linear recording below middle grey (S-Log 3 actually reserves slightly more data for the shadows). BUT that’s for 16 bit. Sony’s 12 bit raw is squeezing 14 stops into what should be an impossibly small number of code values, so in practice what I have found is that 10 bit S-log has noticeably more data in the shadows than 12 bit raw.
In the highlights 12 bit linear raw will have more data than 10 bit S-log2 and S-Log3 and this is borne out in practice where a brightly exposed raw image will give amazing results with beautiful highlights and mid range. But if your 12 bit raw is dark or underexposed it is not going to perform as well as you might expect. For dark and low key scenes 10 bit S-Log is most likely going to give a noticeably better image. (Note: 8 bit S-log2/3 as you would have from an FS5 in UHD only has a quarter of the data that 10 bit has. The FS5 records the first 8 stops in 8 bit S-log 2 with approx 64 code values, S-Log3 is only marginally better at approx 80 code values. 12 bit linear outperforms 8 bit log across the entire range).
Sony’s F5 and F55 cameras record to the R5 and R7 recorders using 16 bit linear data. 16 bit data is enough for 14 stops. But I believe that Sony still use floating point math for 16 bit recording. This time instead of using the floating point math to make room for an otherwise impossible dynamic range they use it to take a little bit of data from the brightest stop to give extra code values in the shadows. When you have 16,384 code values to play with you can afford to do that. This then adds a lot of extra tonal values and shades to the shadows compared to 10 bit log and as a result 16 bit linear raw will outperform 10 bit log across the entire exposure range by a fairly large margin.
So there you have it. I know it’s hugely confusing sometimes. Not all types of raw are created equal. It’s really important to understand this stuff if you’re buying a camera. Just because it has raw it doesn’t necessarily mean an automatic improvement in image quality in every shooting situation. Log can be just as good or possibly even better in some situations, raw better in others. There are reasons why cameras like the F5/R5 cost more than a FS5/Shogun/Odyssey.
I’m running some workshops for Singapore Media Academy in September. Spaces are limited and I don’t get to visit Asia as much as I used to. So if you are interested in attending one of my highly regarded and popular workshops here is a great opportunity.
CREATIVE COMPOSITION:
The first is on Creative Composition. Good shot composition can make or break a production. In the workshop I will guide you through ways to achieve images that will draw the audience in, focus the viewers attention or create different emotional reactions. I will show you how to deal with composition in moving shots, something that can be difficult. We will use case studies of well known movies to see clever use of classic techniques such as the rule of thirds, vanishing lines or Fibonacci curves. We will see how subtle lighting changes can be used to direct the audiences gaze. From the framing of a simple interview to the staging of a complex scene the workshop will help you develop engaging and interesting images. Understanding basic composition is one of the keys to great productions whether it’s for TV news or the cinema.
DIGITAL CINEMATOGRAPHY, LOG AND RAW (Plus HDR).
The second workshop is on Digital Cinematography, Log and Raw. This is an essential workshop for those serious about obtaining the best possible images with a modern electronic video camera. I will explain the differences between standard gammas, log and raw in a way that’s easy to understand but will give you all the information you need to be able to make informed decisions about which to use and when. Next I will teach you how to expose Log and Raw looking closely at how to use exposure offsets for the best results in differing lighting conditions. Then we will look at Look Up Tables. I’ll show you how and when to use them and how to easily create your own. We will finish with some practical end to end workflow sessions where you will develop your own LUT’s, shoot with log or raw and then perform a basic grade on your content. This will include how to use color managed grading tools such as the Academy of Motion Pictures workflow “ACES”. There’s no hard to understand mathematics, no complex formula’s, just easy to understand explanations and great practical tips and advice that will make log and raw easy to understand and use. I will also include how to expose and shoot for HDR and what you need to consider for HDR productions. More details can be found with the links below:
Here’s a little experiment for you to try if you have a PMW-F5, PMW-F55 or PMW-FS7. It should help you understand a few key things about the way these cameras behave, notably:
1: Why ISO does not actually reflect the sensitivity of the camera.
2: Why it is beneficial to expose S-Log2 or S-Log3 brighter than the Sony recommended levels.
3: How to get the best possible S-Log footage.
4: Why S-log may be a poor choice for low light.
Ideally you will want to use an external waveform monitor connected to the cameras SDI output, but it is possible to use the built in waveform display.
Start with the camera in Custom mode. Choose “STD” gamma and Rec-709. Set the gain/ISO settings so that the camera is showing ISO.
Set the ISO to the base ISO (800 ISO on F5/FS7, 500 ISO on F55).
Expose a 90% white card so that white is 90% on the waveform display. This doesn’t need to be 100% accurate, you can use a piece of paper if you don’t have a proper white card. Don’t change the ISO/Gain, light the white card if you need to. Make a note of the aperture.
Now change the gamma selection to S-Log2, do not change the exposure.
Note how white now drops down to about 70% and also note that the ISO becomes 2000 ISO on an F5 or FS7 and 1250 ISO on an F55.
Think about this for a moment: If the ISO has gone up, how can white and the bulk of my image become darker?
Now switch the camera to show dB gain instead of ISO, the gain should be showing 0dB. Repeat the above switching from Standard 709 gamma to S-Log2 and note that the gain remains at 0dB for both rec-709 and S-Log2.
Think about this: The gain is the same for both 709 and S-log2 but the S-Log2 image is darker. As the gain is NOT changing then the sensitivity is not actually changing, so why does the ISO change?
If you were to use a light meter and start off with the light meter set to 800 (500) ISO the light meter would tell you to set the aperture to whatever it is you currently have to give the correct exposure in rec-709 with white at 90%. If you had a light meter and you change the ISO setting on the light meter from 800(500)ISO to 2000(1250) ISO the light meter will tell you to close the aperture by 1.3 stops.
So, on your camera, while it is set to S-Log2 close the aperture from it’s original setting by 1.3 stops. Now you will find that white will be at the recording levels given by Sony for S-Log2 which is 59% for white and 32% for middle grey.
So what have we learnt from this? The gain is the same for both standard gamma and S-Log2, even though the ISO changes from 800(500) to 2000(1250) ISO. So the sensitivity and amount of noise coming from the sensor is the same in both cases. But the indicated ISO changes so that if you are using an external light meter, when you switch to S-Log the higher indicated ISO will make the light meter tell you to close the aperture. This means there is less light falling on the sensor. This means that the recorded image will have a worse signal to noise ratio (noise remains the same, but signal is smaller).
To solution of course to this poorer signal to noise ratio is simply to open the aperture back up again by 1.3 stops. When shooting S-Log2 or S-Log3 using the CineEI mode I always recommend using 800EI on an F5 or FS7 or 640EI on an F55. This means your aperture becomes the same as it would be when shooting in vanilla Rec-709, the end result is the same, improved, signal to noise ratio. If you are not using CineEI or LUT’s, then expose white at 70%.
The Signal to Noise ratio is one of the key factors in determining the quality of a video or stills image. A noisy, grainy picture rarely looks as good as a low noise “clean” image. In addition it’s noise in your images will limit how far you can grade them before the picture quality becomes unacceptably poor.
Almost always what you want is the biggest possible signal with the least possible noise. In a video or film camera the signal is the desired image information or in simple terms the picture. While the noise is…. well….. the noise.
Once upon a time, when film cameras were normal for both still photography and film the noise in the pictures came primarily from the grain structure of the film stock. One of the great features of film cameras is that you can actually change that film stock to suit the type of scene that you are shooting. For low light you could use a more sensitive film stock that was actually truly more sensitive to light. However, often a very sensitive film stock will show more noise as the grain of more sensitive film is normally larger.
With video and digital stills cameras however things are quite different. You can’t normally change the sensor in a video camera and it’s the sensor that determines the sensitivity of the camera and it is the sensor that is the source of the majority of the noise.
Modern CMOS video sensors consist of two parts. The light gathering part and the readout part. The size of the pixels on a sensor is one of the key factors in determining the sensitivity and dynamic range. Small pixels are not good at capturing, converting and storing large numbers of photons of light or electrons of electricity. Bigger pixels are much better at this, so big pixels typically mean better sensitivity and a better dynamic range. Each pixel is unique and as a result every pixel on the sensor will perform slightly differently. The signal stored in the pixels is a tiny analog signal that is easily disturbed by stray electric currents and variations in temperature. As a result of the small variations from pixel to pixel, the stray signals and heat, there is a small variation from moment to moment in the signal that comes off the pixel when it is read out and these variations are what we see as noise.
The analog signal from the pixels is passed to a circuit that converts it to a digital signal. The analog to digital conversion process normally includes some form of noise reduction circuitry to help minimise the noise. By carefully mapping the A to D circuity to the signal range the pixels provide, a sensor manufacturer can find the best combination of noise, dynamic and sensitivity. Once the signal has been converted to a digital one, the noise level, sensitivity and dynamic range is more or less locked in and can’t be changed (Some cameras have the ability to use slightly different A to D conversion ranges to help give improved noise levels at different brightness/dynamic ranges).
The bottom line of all this is that with the vast majority of video cameras the noise level is more or less fixed, as is the sensitivity as we can’t actually swap out the sensor.
But wait! I hear you say…. My camera allows me to change the ISO or gain. Well yes it probably does and in both cases, ISO or gain, with a digital video or stills camera what you are changing is the cameras internal signal amplification. You are NOT making the camera more sensitive, you are simply turning up the volume. As anyone with any type of sound system will know, when you turn up the gain you get more hiss. This is because gain makes not only the desired signal bigger but also the noise. As a result adding gain or increasing the ISO is rarely a great thing to do.
So normally we want to use a digital camera at it’s native sensitivity wherever possible. The native sensitivity is where no gain is being added by the camera or 0dB. In ISO, well you need to find out what the native ISO is and be aware that different gamma curves will have different base ISO’s (which is why I prefer to use dB gain as 0dB = native sensitivity, least noise, best dynamic range, no matter what gamma curve).
To get the best possible image we then want to make our signal (picture information) as big as possible. As we can’t swap out the sensor, the only way to do that is to put as much light as possible onto the sensor. Obviously we don’t want to overload the sensor or exceed the limitations of the recording system, but generally the more light you get on the sensor, the better your pictures will be.
As the sensors noise output remains more or less constant, the best signal to noise ratio will be gained when you put a lot of light on the sensor. This generates a very large signal, so the signal becomes big compared to the noise and the noise becomes only a small percentage of the overall image.
If we are unable to get enough light onto the sensor to expose it fully then it is often tempting to add some gain to make the picture brighter. 6dB of gain is the equivalent to 1 stop of exposure. Just like f-stops, each time we go up a stop we are doubling. So adding 6db of gain doubles everything. It makes the picture the equivalent of one stop brighter, but it also doubles the noise. Adding 12dB gain multiplies the noise 4 times, adding 18dB multiplies the noise 8 times.
What if instead of adding gain to make the picture brighter we let 4x more light fall on the sensor (2 stops)? Well the image gets brighter by the equivalent of 2 stops but as we are not adding gain this means the desirable signal, the picture is now going to be the equivalent of 12dB bigger than the noise than it was before we added the 2 stops of light. That’s going to give you a much cleaner looking image.
How do you get more light onto the sensor? There are many ways such as using a faster lens with a larger aperture that will let more light through. Or you could try using a slower shutter speed (I often find it beneficial in low light to use a 1/24th or 1/25th shutter if there is not too much motion to cause the image to become excessively blurred). Then of course you can also add light to your scene by lighting it. It’s very rare to find noisy and grainy night scenes in feature films and that’s because the night scenes normally have well lit foregrounds but keep dark backgrounds to maintain the sensation of night time or darkness. High contrast is the key to good looking night scenes, well lit foregrounds or actors with deep, dark shadows and backgrounds.
The desire to have a good signal to noise ratio is one of the reasons why when shooting in log or raw you want to expose as brightly as you can (while still maintaining consistent exposure from shot to shot, scene to scene). It’s a little bit harder with standard gammas as we have things like the knee or highlight roll off to deal with. Plus the need to have a shot that looks correct straight out of the camera. But at the end of the day the best results are almost always gained when the gain is kept to a minimum (but don’t use negative gain as this can effect the dynamic range) and the amount of light falling on the sensor as high as possible.
In the next article I’ll give you an interesting experiment to try on a PMW-F5, F55 or PMW-FS7 that is very revealing about the way ISO, gain, exposure and noise behaves that will show why exposing log or raw at +1 to +2 stops is so important.
I’ve been doing a lot of work on shooting SLog-2 with the A7s. I realised almost straight away that a set of LUT’s for this camera would really help speed up my grading and testing. In addition as the camera is only 8 bit I have found that I am actually getting the best results from the Slog-2 if I over expose it just a little bit, depending of course on the scene. So I created a set of LUT’s that includes compensation for shooting at the nominal correct exposure as well as either 1 or 2 stops over exposed. In all there are 20 LUT’s in two sets. One is Rec-709 based LUT’s and the other Filmic LUT’s to act as starting points for further grading.
I am in the process of creating the complete workflow and SLog-2 guide for the A7s which I should be publishing later next week which will have much more information on how to use these LUT’s. But in the mean time here are the LUT’s if anyone want’s to play (and I would like feedback on what you think of them).
The LUT naming goes something like this:
AC A7S 709(800) ZERO
AC (That’s me!). A7S (The Camera).
709(800) = Output gamma or style. ZERO = Exposure off set.
The exposure offset refers to the number of stops the footage is over exposed by relative to the normal SLog2 exposure level of Middle Grey at 32% and 90% white at 59%. ZERO menas no exposure offset. 1STOP would be used when the SLog2 was exposed 1STOP over and in this case the LUT then shifts the exposure back down 1 stop to compensate.
709 = Vanilla Rec-709, very contrasty, but limited highlight response and hard clip of over exposure.
709(800) = Rec-709 gamma with 800% (high) dynamic range. Will be slightly low contrast but deals much better with over exposure or bright highlights than vanilla 709.
Film-Like1 = An extended range gamma with highlight roll off (+400% range), slightly de-saturated, slightly more film like color (small red/yellow removed).
Filmic2 = Extended range low contrast gamma with very good over exposure handling. Slightly de-saturated. Good grading start point.
Filmic3 = Similar to Filmic2 but a little more contrast at the expense of a little less highlight roll off.
Click on the links below to download the LUT sets. PLEASE DO NOT HOST THESE ELSEWHERE OR DISTRIBUTE THESE ELSEWHERE OR VIA ANY MEANS OTHER THAN A LINK TO THIS PAGE.
There is an ongoing and much heated debate on another forum about the practicalities of using the LUT’s or Looks built in to the PMW-F5 and PMW-F55 for setting the correct exposure of your SLog or Raw footage. In response to this I put together a very rough video demonstrating how this actually works.
Before watching the video, do please understand the following notes:
Correct exposure is normally determined by the level at which middle grey is recorded. This is true of both video and film production. Light meters are calibrated using middle grey. Expose with a light meter and you will find middle grey at the levels indicated below.
Different gamma curves may use different middle grey levels depending on the contrast required and the dynamic range of the gamma curve. Generally speaking, the greater the dynamic range, the lower middle grey must be set in order to leave room above middle grey for the extra dynamic range. This means that the relationship between middle grey and white will be different from curve to curve. Don’t always expect white to be some fixed value above middle grey. Some of the Sony looks for example LC709TypeA are very low contrast and while middle grey still sits at around 42% (The ITU standard for Rec-709 is 41.7%), because it is a low contrast, high dynamic range curve white is at a lower level, around 70%. The Hypergamma LUT grey points are given by the “G40″ or G33” number – G40 meaning middle grey at 40%.
When you take Slog or raw in to post production it is expected that the middle grey of the recordings will be at the correct nominal level (see chart below). If it is not, when you apply a post production Slog or raw LUT then the footage may appear incorrectly exposed. If you try to bring Slog or raw into an ACES workflow then ACES expects middle grey to be at the correct values. So it is important that your Slog or raw is exposed correctly if you want it to work as expected in post.
Correct exposure levels for Sony’s Slog.
Having said all of the above… If you are using CineEI and lowering or raising the EI gain from the native ISO then your Slog or raw will be exposed brighter or darker than the levels above. But I must assume that this is what you want as you are probably looking to adjust the levels in post to reduce noise or cope with an over exposure issue. You may need to use a correction LUT to bring your Slog levels back to the nominal correct levels prior to adding a post production LUT.
These two words, latitude and dynamic range are often confused and are often used interchangeably. Sometimes they can be the same thing (although rare), sometimes they may be completely different. So what is the difference and why do you have to be careful to use the right term.
Lets start with dynamic range as this is the simplest to understand. When talking about a digital camera the dynamic range is quite simply the total range from the darkest shadow to the brightest highlight that the camera can resolve in a single shot. To be included in the dynamic range you must be able to discern visually or measure with a scope a brightness change at both ends of the range. So a camera that can resolve 14 stops will be able to shoot a scene with a 14 stop brightness range and show some information from stop 0 to stop 14. It is not just a measure of the cameras highlight handling, it includes both highlights and shadows. One camera may be very low noise, so see very far into the shadows but not be so good with highlights. While another may be noisy, so not able to see so far into the shadows but have excellent highlight handling. Despite these differences both might have the same dynamic range as it is the range we are looking at, not just one end or the other.
One note of caution with published dynamic range figures or measurements is that while you may be able to discern some picture information in those deepest shadows or brightest highlights, just how useable both ends of the range are will depend on just how the camera performs at it’s extremes. It is not uncommon for the darkest stop to be so close to the cameras noise floor that in reality it’s barely useable, but as it can be measured it will be included in the manufacturers dynamic range figures.
This brings us on to latitude because latitude is a measure of just how flexible you can be with your exposure without significantly compromising the finished picture. The latitude will always be less than the cameras dynamic range. With a film camera, the film stock would have a sensitivity value or ISO. You would then use an exposure meter to determine the optimum exposure. The latitude would then be how much can you over expose or under expose and still have an acceptable result. But what is “an acceptable result”? Here is one of the key problems with determining latitude, what some people may find unacceptable others may be happy with so it can be difficult to quantify the exact latitude of a film stock or video camera precisely. However what you can do is determine which cameras have bigger ranges for example camera “A” has a stop more latitude than camera “B” provide you use a consistent “acceptable quality” assesment.
Anyone that’s shot with a traditional ENG or home video camera will know that you really need to get your exposure right to get a decent looking picture. Take a simple interview shot, expose it right and it looks fine. Overexpose by 1 stop and it looks bad, even if you try to grade it it will still look bad. So in this example the camera would have less than 1 stop of over exposure latitude. But if you underexpose a video camera, the picture gets darker, but after a bit of work in post production it may well still look OK. It will depend in most cases on how noisy the picture becomes when you boost the levels in post to brighten the picture. But typically you might be able to go 1 to 1.5 stops under exposed and still have a useable image. So in this case the camera would have 1.5 stops of underexposure latitude. This then gives a total latitude for our hypothetical camera of around 2 to 2.5 stops.
But what of we increase the dynamic range of the camera or have a camera with a very big dynamic range.Does my latitude increase?
Well the answer is maybe. In some cases the latitude may actually decrease. How can that be possible, surely with a bigger dynamic range my latitude must be greater?
Well, unless your shooting linear raw (more on that in a bit) you will be using some kind of gamma curve. The gamma curve is there to allow you to squeeze a large dynamic range into a small amount of data. It does this by mimicking the way we perceive light in a non linear manner and uses less data in highlights which are perceptually less important to us humans. Even uncompressed video normally has a gamma curve. Without a gamma curve the amount of data needed to record a decent looking picture would be huge as every additional stop of dynamic range actually needs twice as much data as the previous to be recorded faithfully.
With cameras with larger dynamic ranges then things such as knee compression or special gamma curves like Hypergamma, Cinegamma or Log are used. The critical thing with all of these is that the only way to squeeze that greater dynamic range into the same size recording bucket is by adding extra compression to the recorded image.
This compression is normally restricted to the highlights (which are perceptually less important). Highlight compression now presents us with an exposure problem, because if we over expose the shot then the picture won’t look good due to the compression. This means that even though we might have increased the cameras dynamic range (by squeezing and compressing more information into the highlight range) we may have reduced the exposure latitude as any over exposure places important mid range information into the highly compressed part of the gamma curve. So bigger dynamic range does not mean greater latitude, in fact in many cases it means less latitude.
Here’s the thing. Unless you make the recording data bucket significantly bigger (better codec and more bits, 10 bit 12 bit etc), you can’t put more data (dynamic range or stops) into that bucket without it overflowing or without squashing it. Given that most cameras used fixed 8 bit or 10 bit recording there is a finite limit to what can be squeezed into the codec without making some pretty big compromises.
Compression point with Hypergamma/Cinegamma.
With a standard gamma curve white is exposed around 90% to 95%, remember a white card only reflects 90% of the light falling on it not 100%. Middle grey perceptually appears half way between black and white so it’s around 40%-45%. Above 90% is where the knee normally acts to compress highlights to squeeze quite a large dynamic range into a very small recording range, so anything above 90% will be very highly compressed, but below 90% we are OK and we can safely use the full range up to 90%. Expose a face below 90% and it will look natural, above 90% it will look washed out, low contrast and generally nasty due to the squeezing together of the contrast and dynamic range.
But what about a Hypergamma or Cinegamma (or any other high dynamic range gamma curve)? Well these don’t have a knee, instead they start to gradually introduce compression much lower down the gamma curve. A little bit at first and then ever increasing amounts as we go up the exposure range. This allows them to squeeze in a much greater dynamic range in a pleasing way (provided you expose right). But this means that we can’t afford to let faces etc go as high as with the standard gamma because if we do they will start to creep in to the highly compressed part of the curve. So this means that even the slightest over exposure will hurt our image. So even thought they have greater dynamic range, these curves have less exposure latitude because we really really can’t afford to over expose them. Sony compensate for this to some degree by recommending a lower middle grey point between 32 and 40% depending on the curve you use. This then brings your overall exposure lower so your less likely to over expose, but that now means you have less under exposure range as your already shooting a bit darker (White with the hypergammas tends to fall lower, around 80%, so faces and skin tones that would normally be around 70% will be around 60%).
More highlight compression means exposure is still critical despite greater dynamic range
But what about Log?
Now lets look at S-Log2, S-log3. Most log curves are also similar, very highly compressed gamma curves with huge amounts of highlight compression to squeeze in an exceptionally large dynamic range. With Slog2 White is designed to be at 59% and middle grey at 32% and with S-log3 middle grey is 41% and white 61%. So faces will need to sit between around 40% and 50% to look their best. Now log is a little bit different. Log shooting is designed to be done in conjunction with LUT’s (Look Up Tables) in post production. These LUT’s convert the signal from Log gamma to conventional gamma. When you apply the correct LUT to correctly exposed Log everything comes out looking good. What about over exposed Log? This is where it can get tricky. If you have a good exposure correction LUT or really know how to grade log properly (which can be tricky) then you can expose Log by one or 2 stops, but no more (in my opinion at least, 2 stops is a lot of over exposure for Log, I would try to stay less than 2 stops over). Over expose too much and the image gets really hard to grade and may start to lack contrast. One thing to note is when I say over-exposed with respect to log, I’m not talking about about a clipped picture, but simply an image much brighter than it should be. For example with Slog3 faces will be around 52%. If you expose faces at 70% your actually just over 2 stops over exposed and grading is going to start to get tricky and you may find it hard to get your skin tones just right. So, when shooting log make sure you know what the recommended levels are for the curve you are using. I’m not saying you can’t over expose a bit, just be aware of what is correct and that level shifts of just a 7 or 8% may represent a whole stop of exposure change.
It’s only when you stop shooting with conventional gamma curves and start shooting linear that the latitude really starts to open up. Cameras like the Sony F5/F55 use linear raw recording that does not have a gamma curve. When you have no gamma curve then there is no highlight compression. So for example you could expose a face anywhere between in conventional terms between say 45% (the point where perhaps it becomes too noisy if you expose any darker) and 100% it will look just fine after grading because at no point does it become compressed. This is a massive latitude increase over a camera using a gamma curve. It gets even better if the camera is very low noise as you can afford to expose at an even lower level and bring it up in post. This is why raw is such a big deal. I find it much easier to work with and grade raw than log because raw just behaves nicely.
In Conclusion:
Dynamic range is the range the camera can see from the deepest darkest shadows to the brightest highlights in the same shot. Latitude is the range within the dynamic range where we can expose and still get a useable image.
A camera with lower noise will allow you to expose darker and bring your levels up in post, this gives an increase in under exposure range.
Most video cameras have a very limited over exposure latitude due to aggressive highlight compression. This is the opposite to a film camera.
Bigger dynamic range does not always mean greater latitude.
Cameras that shoot raw typically have a much greater latitude than a camera shooting with a gamma curve. For example an F5 shooting SLog2/3 has a much smaller exposure latitude than when shooting raw even though the dynamic range is the same in both cases.
One of the most common questions I get asked is “which gamma curve should I use?”.
Well it’s not an easy one to answer because it will depend on many things. There is no one-fits-all gamma curve. Different gamma curves offer different contrast and dynamic ranges.
So why not just use the gamma curve with the greatest dynamic range, maybe log? Log and S-Log are also gamma curves but even if you have Log or S-Log it’s not always going to be the best gamma to use. You see the problem is this: You have a limited size recording bucket into which you must fit all your data. Your data bucket, codec or recording medium will also effect your gamma choice.
If your shooting and recording with an 8 bit camera, anything that uses AVCHD or Mpeg 2 (including XDCAM), then you have 235 bits of data to record your signal. A 10 bit camera or 10 bit external recorder does a bit better with around 940 bits of data, but even so, it’s a limited size data bucket. The more dynamic range you try to record, the less data you will be using to record each stop. Lets take an 8 bit camera for example, try to record 8 stops and that’s about 30 bits per stop. Try to extend that dynamic range out to 11 stops and now you only have about 21 bits per stop. It’s not quite as simple as this as the more advanced gamma curves like hypergammas, cinegammas and S-Log all allocate more data to the mid range and less to highlights, but the greater the dynamic range you try to capture, the less recorded information there will be for each stop.
In a perfect world you would choose the gamma you use to match each scene you shoot. If shooting in a studio where you can control the lighting then it makes a lot of sense to use a standard gamma (no knee or knee off) with a range of up to 7 stops and then light your scene to suit. That way you are maximising the data per stop. Not only will this look good straight out of the camera, but it will also grade well provided your not over exposed.
However the real world is not always contained in a 7 stop range, so you often need to use a gamma with a greater dynamic range. If your going direct to air or will not be grading then the first consideration will be a standard gamma (Rec709 for HD) with a knee. The knee adds compression to just the highlights and extends the over-exposure range by up to 2 or 3 stops depending on the dynamic range of the camera. The problem with the knee is that because it’s either on or off, compressed or not compressed it can look quite electronic and it’s one of the dead giveaways of video over film.
If you don’t like the look of the knee yet still need a greater dynamic range, then there are the various extended range gammas like Cinegamma, Hypergamma or Cinestyle. These extend the dynamic range by compressing highlights, but unlike the knee, the amount of compression starts gradually and get progressively greater. This tends to look more film like than the on/off knee as it tends to roll off highlights much more gently. But, to get this gentle roll-off the compression starts lower in the exposure range so you have to be very careful not to over expose your mid-range as this can push faces and skin tones etc into the compressed part of the curve and things won’t look good. Another consideration is that as you are now moving away from the gamma used for display in most TV’s and monitors the pictures will be a little flat so a slight grade often helps with these extended gammas.
Finally we come to log gammas like S-Log, C-Log etc. These are a long way from display gamma, so will need to be graded to like right. In addition they are adding a lot of compression (log compression) to the image so exposure becomes super critical. Normally you’ll find the specified recording levels for middle grey and white to be much lower with log gammas than conventional gammas. White with S-Log for example should only be exposed at 68%. The reason for this is the extreme amount of mid to highlight compression, so your mid range needs to be recorded lower to keep it out of the heavily compressed part of the log gamma curve. Skin tones with log are often in the 40 – 50% range compared to the 60-70% range commonly used with standard gammas. Log curves do normally provide the very best dynamic range (apart from raw), but they will need grading and ideally you want to grade log footage in a dedicated grading package that supports log corrections. If you grade log in your edit suite using linear (normal gamma) effects your end results won’t be as good as they could be. The other thing with log is now your recording anything up to 13 or 14 stops of dynamic range. With an 8 bit codec that’s only 17 – 18 bits per stop, which really isn’t a lot, so for log really you want to be recording with a very high quality 10 bit codec and possibly an external recorder. Remember with a standard gamma your over 30 bits per stop, now were looking at almost half that with log!
Shooting flat: There is a lot of talk about shooting flat. Some of this comes from people that have seen high dynamic range images from cameras with S-Log or similar which do look very flat. You see, the bigger the captured dynamic range the flatter the images will look. Consider this: On a TV, with a camera with a 6 stop range, the brightest thing the camera can capture will appear as white and the darkest as black. There will be 5 stops between white and black. Now shoot the same scene with a camera with a 12 stop range and show it on the same TV. Again the brightest is white and black is black, but the original 6 stops that the first camera was able to capture are now only being shown using half of the available brightness range of the TV as the new camera is capturing 12 stops in total, so the first 6 stops will now have only half the maximum display contrast. The pictures would look flatter. If a camera truly has greater dynamic range then in general you will get a flatter looking image, but it’s also possible to get a flat looking picture by raising the black level or reducing the white level. In this case the picture looks flat, but in reality has no more dynamic range than the original. Be very careful of modified gammas said to give a flat look and greater dynamic range from cameras that otherwise don’t have great DR. Often these flat gammas don’t increase the true dynamic range, they just make a flat picture with raised blacks which results in less data being assigned to the mid range and as a result less pleasing finished images.
So the key points to consider are:
Where you can control your lighting, consider using standard gamma.
The bigger the dynamic range you try to capture, the less information per stop you will be recording.
The further you deviate from standard gamma, the more likely the need to grade the footage.
The bigger the dynamic range, the more compressed the gamma curve, the more critical accurate mid range exposure becomes.
There is a lot of hype around shooting flat. Shooting flat has become a fashionable way to shoot and many individuals and companies have released camera settings said to provide the flattest images or to maximise the camera dynamic range. Don’t get me wrong, I’m not saying that shooting flat is necessarily wrong or that you shouldn’t shoot flat, but you do need to understand the compromises that can result from shooting flat.
First of all what is meant by shooting flat? The term comes from the fact that images shot flat look, err, well…. flat when viewed on a standard TV or monitor. They have low contrast and may often look milky or washed out. Why is this? Well most TV’s and monitors only have a contrast range that is the equivalent of about 6 stops. (Even a state of the art OLED monitor only has a range of about 10 to 11 stops). The whole way we broadcast and distribute video is based on this 6 stop range. The majority of HD TV’s and monitors use a gamma curve based on REC-709, which also only has a 6 to 7 stop range. Our own visual system has a dynamic range of up to 20 stops (there is a lot of debate over exactly how big the range really is and in bright light our dynamic range drops significantly). So we can see a bigger range than most TV’s can show, so we can see bright clouds in the sky as well as deep shadows while a TV would struggle to show the same scene.
Modern camera sensors have dynamic ranges larger than 6 stops, so we can almost always capture a greater dynamic range than the average monitor can show. Now consider this carefully: If you capture a scene with a 6 stop range and then show that scene on a monitor with a 6 stop range, you will have a very true to life and accurate contrast range. You will have a great looking high contrast image. This is where having matching gammas in the camera and on the monitor comes in to play. Match the camera to the monitor and the pictures will look great, 6 stops in, 6 stops out. But, and it’s a big BUT. Real world scenes very often have a greater range than 6 or 7 stops.
A point to remember here: A TV or monitor has a limited brightness range. It can only ever display at it’s maximum brightness and best darkness. Trying to drive it harder with a bigger signal will not make it any brighter.
Feed the monitor with an image with a 6 stop range and a Rec-709 signal and the monitor will be showing it’s blackest blacks and it’s brightest whites.
But what happens if we simply feed a 6 stop monitor with an 11 stop image? Well it can’t produce a brighter picture so the brightest parts of the displayed scene are no brighter and the darker, no darker so the image you see appears to have the same brightness range but with less contrast as 11 stops are being squeezed into a 6 stop brightness range, it starts to look flat and un-interesting. The bigger the dynamic range you try to show on your 6 stop monitor, the flatter the image will look. Clearly this is undesirable for direct TV broadcasting etc. So what is normally done is to map the first 5 stops from the camera more or less directly to the first 5 stops of the display so that the all important shadows and mid-tones have natural looking contrast. Then take the brighter extended range of the camera, which may be 3 or 4 stops and map those into the remaining 1 or 2 stops of the monitor. This is a form of compression. In most cases we don’t notice it as it is only effecting highlights and our own visual system tends to concentrate on shadows and mid-tones while largely ignoring highlights. This compression is achieved using techniques such as knee compression and is one of the things that gives video it’s distinctive electronic look.
A slightly different approach to just compressing the highlights is to compress much more of the cameras output. Gamma curves like Sony’s cinegammas or hypergammas use compression that gets progressively more aggressive as you go up the exposure range. This allows even greater dynamic ranges to be captured at the expense of a slight lack of contrast in the viewed image. Taking things to the maximum we have gamma curves that use log based compression where each brighter stop is in effect compressed twice as much as the previous one. Log gamma curves like S-Log or Log-C are capable of capturing massive dynamic ranges of anywhere up to 14 stops. View these log compressed images back on your conventional TV or monitor and because even the mid range is highly compressed they will look very low contrast and very flat indeed.
Note: Log gamma does not actually increase compression, in fact it allocates exactly the sane amount of data to every stop of exposure. However it must be remembered that for every stop you go up in exposure the brightness of the scene becomes 2 times brighter. So to record the scene accurately you should use twice as much data for every stop you add. But Log does not do this, it just adds a small amount of extra data. Thus in effect RELATIVE TO THE BRIGHTNESS RANGE OF THE SCENE the amount of data is halved for each stop you go up in exposure.
So, if you have followed this article so far you should understand that we can capture a greater dynamic range than most monitors can display, but when doing so the image looks un-interesting and flat.
So, if the images look bad, why do it? The benefits of capturing a big dynamic range are that highlights are less likely to look over exposed and your final image contrast can be adjusted in post production. These are the reasons why it is seen as desirable to shoot flat.
But there are several catches. One is that the amount of image noise that the camera produces will limit how far you can manipulate your image in post production. The codec that you use to record your pictures may also limit how much you can manipulate your image due to compression artefacts such as banding or blocking. Another is that it is quite easy to create a camera profile or setup that produces a flat looking image, for example by artificially raising the shadows, that superficially looks like a flat, high dynamic range image, but doesn’t actually provide a greater dynamic range, all that’s happened is that shadows have been made brighter but no extra dynamic range has actually been gained.
Of course there are different degrees of flat. There is super flat log style shooting as well as intermediate flat-ish cinegamma or hypergamma shooting. But it if you are going to shoot flat it is vital that the recorded image coming from the camera will stand up to the kind of post production manipulation you wish to apply to it. This is especially important when using highly compressed codecs.
When you use a high compression codec it adds noise to the image, this is in addition to any sensor noise etc. If you create a look in camera, the additional compression noise is added after the look has been created. As the look has been set, the compression noise is not really going to change as you won’t be making big changes to the image. But if you shoot flat, when you start manipulating the image the compression noise gets pushed, shoved and stretched, this can lead to degradation of the image compared to creating the look in camera. In addition you need more data to record a bigger dynamic range, so a very flat (wide dynamic range) image may be pushing the codec very hard resulting in even more compression noise and artefacts.
So if you do want to shoot flat you need a camera with very low noise. You also need a robust codec, preferably 10 bit (10 bit has more data levels than 8 bit so contains more tonal information) and you need to ensure that the camera setup or gamma is truly capturing a greater dynamic range, otherwise your really wasting your time.
Shooting flat is a great tool in the cinematographers tool box and with the right equipment can bring great benefits in post production flexibility. Most of the modern large sensor cameras with their low noise sensors and ability to record to high end 10 bit codecs either internally or externally are excellent tools for shooting flat. But small sensor cameras with their higher noise levels do not make the best candidates for shooting flat. In many cases a better result will be obtained by creating your desired look in camera. Or at least getting close to the desired look in camera and then just tweaking and fine tuning the look in post.
As always, test your workflow. Just because so and so shoots flat with camera A, it doesn’t mean that you will get the same result with camera B. Shoot a test before committing to shooting flat on a project, especially if the camera isn’t specifically designed and set up for flat shooting. Shooting flat will not turn a poor cinematographer into a great cinematographer, in fact it may make it harder for a less experienced operator as hitting the cameras exposure sweet spot can be harder and focussing is trickier when you have a flat low contrast image.
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