Tag Archives: picture

Choosing the right gamma curve.

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.

Flat isn’t always better.

Picture Profiles to match PMW-F3 and NEX-FS700

IMPORTANT PLEASE ENSURE YOU USE THE REVISED SETTINGS UPDATED ON 24th  JULY.

After my recent side by side look at the F3 and FS700 and seeing how different the two cameras look, I decided to try to match them a bit better. There will be many shoots where I will use them both together so getting them to look the same is important. I thought this would be a relatively straight forward task, simply dial in the FS700 to match the F3.

Well it wasn’t simple and it ended up taking me several hours to get to the point where I couldn’t get them any closer. The main issues are that the F3, like most of the XDCAM cameras has a yellow colour cast that’s hard to completely remove and the FS700 has quite a blue image and only very limited matrix controls. Initially I started to try to match the FS700 to a standard F3. While I could get the FS700 closer to the F3, I just couldn’t get a near match let alone a complete match. So back to the drawing board.

For my second attempt I decided first to work on getting rid of the yellow/orange cast to the F3 pictures by adjusting the F3’s matrix, at the same time creating a neutral look picture profile with good dynamic range, but one that could be used without grading. This took some extensive matrix tweaks. You will find the full details of my new “STD-REAL” picture profile in the forum by clicking here.

So once I had a neutral starting point on the F3 I then turned to the FS700 which I think is very blue. The matrix settings on the FS700 are quite limited so I wasn’t able to get an exact match to the F3, however the setting I came up with get them close enough for most jobs, it’s not perfect but it will do. I’m quite happy with my new FS700 settings and I think with this profile it produces a very nice image. You can find the full profile settings in the forum by clicking here. Remember you need to use the matching F3 profile in the F3 for the best match. If you want the maximum dynamic range then instead of Cinegamma 1 you should use Cinegamma 4 with the black gamma set to zero. My STD REAL profile for the FS700 is closer to a standard F3 than the default FS700 settings.

Low Light Picture Profile for EX1/EX3

I get asked a lot about settings for shooting in low light with the EX1 and EX3. To be honest there is not much that will make a big difference that can be done, beyond adding in camera gain. There are a few tweaks you can make to the picture profiles that will help minimise noise levels and give a slightly brighter picture without resorting to overall gain and I’ll go through those here.

Gamma: By using a brighter or higher gain gamma curve you can get a slightly brighter image without an across the board gain increase. Do however consider though that gamma does add gain so a brighter gamma curve has more gain and thus more noise than a darker gamma curve. Where you light range is limited or controlled then I recommend using Standard Gamma 2 with the black gamma set to +40. Raising the black gamma helps lift shadow and dark areas of the image. For scenes with bright highlights then it’s useful to have some extra dynamic range and in this case I would choose cinegamma 4, again with the black gamma raised, this time to +50.

If you are happy with turning detail off altogether then this may be a wise choice as it will prevent any noise from being enhanced. If not in order to keep the appearance of noise to a minimum I would decrease the detail level to -10. As we are shooting in low light then I will assume there are a lot of dark areas in the image. To keep noise less visible in low contrast areas I would set the crisping to +50.  This will slightly soften the image but help control noise.

There are two principle forms of noise, chroma noise and luma noise. There’s not much we can do about luma noise other than controlling detail enhancement as above, but if we reduce the image colour saturation we can reduce the chroma noise. Better still using the low key sat function we can just reduce the chroma (colour) level in low key parts of the shot. So for my low light profile I would set Low Key Sat to somewhere around -50.

So by changing the gamma we can increase the sensitivity a little, turning off the detail correction or using crispening we can ensure that the visibility of any noise is as minimised and the Low Key Sat function will keep the noise to a manageable level.

These setting won’t turn your EX1 or EX3 into a mega low light monster, but they will give a small boost to the low light performance before you have to resort to adding gain. Talking of gain, do make sure you read this to understand what gain is doing.

EX1/EX3 Picture Profile suggestions for low light:

Gamma Standard 2, Black Gamma +40  OR Cinegamma 4, Black Gamma +50

Detail OFF or Detail Level -10, Crispening +50

Low Key Sat -50

Black level -3 (restores black to zero)

 

Set of Canon C300 Custom Profiles for Download.

I’ve been playing and have come up with my first set of custom profiles for the Canon C300. You’ll find all the details here: http://www.xdcam-user.com/forum3/viewtopic.php?f=44&t=962 including a downloadable package that you can copy to an SD card and the load the files directly to your own C300.

You must be registered with the forum/blog to view the page, but registration is free and open to all. Any problems let me know.

The Profiles include:

AC-Neutral: a natural looking, true to life image.
AC-Vivid: A bright colourful and vivid image.
AC-3200a: Use this profile when shooting at ISO 3200 to maximise sensitivity and control noise.
AC-6400a: Use this profile when shooting at ISO 6400 to maximise sensitivity and control noise.
AC-Cine1: A neutral filmic looking image that can be used straight form the camera or graded.

C300 Moiré

C300 Moire

The C300 is not Moiré free as can be seen from this blown up section of a frame grab. Once again it’s fine brickwork thats causing the problem. Now before everyone runs off in a panic, lets put this into perspective. The F3′ aliases, the Alexa aliases as do most single chip cameras. This is certainly no worse than an F3 and is right at the resolution limits of the camera, so your not going to see it very often. It takes a very fine, high contrast pattern, in sharp focus before you’ll see this kind of thing.

XDCAM Picture Profiles and setups, also C300 coming soon.

I’ve added a new section in the xdcam-user.com forum for listing details of my various picture profiles. You will need to be a registered forum member to view or comment, but registration is free. I hope to add many profiles to this forum over the coming weeks for many of the XDCAM cameras as well as the new Canon C300 once I start to get that dialled in. I’ve started with my EX S-Log style gamma curve.

http://www.xdcam-user.com/forum3/viewtopic.php?f=41&t=194&sid=c233ea884673388efe1e1af8c2ef84c7

S-Log on a non S-Log PMW-F3 and Log on an EX1/EX3

Note: There is something up with the frame grabs. For some reason they are very dark. I’ll look into this in the morning and get some more accurate grabs online.

First of all let me say thanks to Ben Allan on CML list for getting me thinking about this. He has already started experimenting with creating a log style Picture Profile for the EX1. All the setting you’ll find here are my own work and based on tests done with real scenes and some dodgy home made latitude test charts 😉

Ben’s musings on CML made me consider what S-Log is. In essence it is nothing more than a clever gamma curve that allows you to capture a greater dynamic range than is normally possible with conventional gamma curves. The reason why the standard gamma dynamic range is normally constrained is in part simply because if you record too large a dynamic range and then show it on a conventional monitor or TV, it simply does not look right. So to make it look right it must be graded in post production. In order to do a significant grade in post, the quality of the recording has to be good enough to withstand a fair bit of pulling and pushing. As a result 10 bit recording is recommended (however it is still possible to work with lot with top quality low noise 8 bit recordings, not that I would recommend this). Anyway as both the standard PMW-F3 and EX1/EX3 have 10 bit outputs I decided to see if it was possible to come up with a picture profile that would mimic a Log curve and then see if it actually brings any real world advantage.

Genuine S-Log, mid grey @38%

First up I experimented with the F3. I already have the S-Log option, so this gave me a benchmark to work against. To mimic S-Log you need to increase the gamma gain at the lower end of the curve, you can do this with the Black Gamma function. I know that with S-Log the cameras native ISO is 800 as this is the sensitivity at which maximum dynamic range can be realised with the F3’s sensor. So I started my experiments at 800iso. I could bring up the shadow detail with the Black Gamma but I notice that I appeared to be trading off some highlight handling for shadow information, so while the images kind of looked like S-Log, they did not really gain any latitude.

AC-Log v1. Very similar to S-Log, same exposure as S-Log

During this process I realised that my mid range sensitivity was now a lot higher than with genuine S-Log, so I decreased the camera gain so I was now at 400iso and started tweaking again. Now with Black Gamma all the way up at +99 I was seeing around 1 stop further into the shadows, with no impact on highlight handling.

When I tested my new Picture Profile on a real scene, exposing as you would S-Log with mid grey at 38% I was very pleased to find some very similar images that do grade quite well. As well as the Gamma tweaks I also incorporated a few other changes into the profile to increase the overall grade-ability.

CineGamma 4, mid-grey at 38%

There is a definite improvement in shadow reproduction. It’s not as good as real S-Log, but it does give a very useful improvement for those without S-Log. One interesting point is that the exposure between the two log frame grabs posted here is not changed, so even though the camera is set at 400iso, when the picture profile is applied the camera behaves more like an 800iso camera and exposure should be set accordingly.  I think my PP (which you can download at the bottom of the page) brings a little under a one stop improvement in DR, real S-Log is about 2 stops.

If you click on the image captures you can view them full frame. When you compare the AC-Log and Cinegamma 4 images you should be able to see more shadow detail in the tree on the right of frame with the AC-Log yet the sky is further from clipping as well.

So what about the EX1 and EX3, can the same be done for them? Well this is much more of a challenge as the EX cameras are much noisier. Simply bringing up the Black Gamma does help you see into the shadows a bit better but it comes at the cost of a lot of extra noise and really makes it un gradable. Normally I don’t recommend using negative gain as it can reduce the dynamic range of the camera. But I figured if I use negative gain and then increase the gamma gain that should cancel out any dynamic range loss. To then avoid the usual -3db reduction in highlight performance I adjusted the overall gamma gain to return the peak output level to 109IRE. After a bit of fiddling around with my test charts and waveform monitors I could see that it was possible to gain a small amount of dynamic range, a little under 1 stop, however there is an overall increase in the noise level of about +4db. Now that doesn’t sound too terrible, but to gain the extra stop of DR you have to under expose compared to standard gamma’s, typically with S-Log you would put mid grey at 38% (use the centre spot meter on the EX1/EX3 and a grey card). This works reasonable well with this fake log picture profile. The problem however is that when grading you may find that you have to add still further gain to bring skin tones to a normal level and this will accentuate the noise. You could use something like the Neat Video plugging to reduce the noise and in this case I think this sudo Log picture profile could be handy in tricky lighting situations. The EX1R Log picture profile, to work correctly MUST be used in conjunction with -3db gain, any other gain setting and you will loose dynamic range. Again like real S-Log, 10 bit external recording is desirable, but why not play with the picture profile and try it for yourself. It is a bit experimental, I’m not convinced that the extra stop of DR is worth the noise penalty on the EX1R, but then I’m spoilt as I have an S-Log F3.

I have uploaded both the F3 and EX1R picture profiles into a single zip file that you can download below. You will need to have an account on xdcam-user.com to download them, or register for a new account first. Un-zip the package and copy the SONY folder to the root of an SxS card, so you should have both a BPAV folder and a SONY folder in the root directory. The cameras will need the latest firmware versions to load the single profile directly. In the Picture Profile menu choose an empty PP and then in the bottom PP menu chose “load”.

[downloads_box title=”F3 and EX1R Log Picture Profiles”]
F3 and EX1R Log like profiles
[/downloads_box]

Picture Profiles – Scene Files – Crispeneing. How to clean up your image.

I promised I would re-visit some of my Picture Profile stuff. I thought I would start with this one as it is one of the least well understood settings. It’s effects are quite subtle, but it can mean the difference between a noisy picture and a clean image, but also between a sharp image and a soft image, in particular in areas of subtle detail or low contrast detail such as foliage, grass and textures.

Crispening is a part of the detail correction circuit. It does not in itself, as it’s name suggests (at least on an EX of F3) make the image “crisper”. What it does is control the contrast range over which the detail circuit operates. Basically it sets the threshold at which detail correction is applied to the image, which in turn can make the image look a little sharper or less sharp. The apparent sharpness itself is controlled by the Detail Level and Frequency controls.

Why is this useful? Well it allows the user to choose whether to opt for a cleaner looking image or a sharper looking image. An important consideration is that this adjustment does not change the actual resolution of the image or the noise level of the camera, but it does make subtle details in the image more or less enhanced and as noise is also a subtle, even if unwanted detail within the image it will also make noise more or less enhanced, thus more or less visible.

Imaginary waveform showing real picture information plus noise.

In the first illustration I have drawn an imaginary video waveform signal coming from the camera that contains a mixture of noise and both subtle and more obvious picture information. The bigger the up/down change in the waveform the more obvious the change in brightness (and thus contrast) on the monitor or TV would be. Throughout the image there is some noise. I have indicated the noise level for the camera with a pair of red lines. The EX1 and EX3 is a moderately noisy camera, not the worst, nor the best for an HD camera, but pretty good in it’s price range. So if we can do something to make the noise less obvious that would be desirable in many cases. Crispening can help us do that. Crispening ONLY has an effect when you are applying detail correction to the image. It sets the threshold at which detail correction is applied. The default setting on an EX is zero.

Crispening threshold set to -60 indicated by green lines.If we reduce the crispening setting, lets say to -60, it REDUCES the threshold at which detail is applied which generally makes the pictures look sharper. Looking at the second and third illustrations you can see how if you reduce the threshold too much then detail correction will be applied to even the most subtle changes in the image, including the image noise. The little black spikes I have added to the diagram illustrate the way the detail “enhancement” will be added to both noise and subtle contrast changes as well as larger contrast changes.

Black spikes represent detail correction being added to real picture information and noise when crisping set to -60.

This will make the pictures look more noisy, but… and this is important… it will also help bring out subtle low contrast textures in foliage, skin, fabrics etc. A area where perhaps the EX1 and EX3 don’t do terribly well.

If you want a clean image however where noise is less visible, then raising the crispening level to a high positive value, lets say +60 will increase the threshold at which detail correction is added, so signal changes will need to be bigger before detail correction is applied.

Much higher crisping threshold when set to +60

With a high positive number the image will look cleaner and less noisy, but you will loose some enhancement in textures and low contrast areas as these will no longer have detail correction applied to them. This can lead to a slightly muddy or textureless look to tress, grass, skin and fabric.

The real problem areas are the subtle textures and low contrast areas (circled in orange) where the true image detail is barely above the noise level. It’s very difficult to bring these out without increasing the appearance of noise. With Crispening at +60 detail correction is only applied to larger brightness or contrast changes within the image, so noise becomes less obvious.Unfortunately there is no clear answer to how to set the crispening level as it will depend on what you are shooting and how much noise you can tolerate. I tend to have crisping set between +10 and +30 for most things as I do tend to do a fair amount of grading work on my footage. When you grade noise is often the limiting factor as to how far you can push the image, so I like to keep noise under control as much as possible. For green screen and chroma key work I push crispening up to +40 to +60 as this helps me get a cleaner key, especially around subtle edges and hair.

Problem areas circled, subtle textures get lost if detail level set too high, although image looks much cleaner.

If I am shooting exteriors and scenics with lots of foliage, grass etc then I will sometimes go down to -30 as this helps bring out the subtle textures in the leaves and plants, but this can make noise a little more pronounced, so it’s a trade off. And that’s what Crispening is all about, trading off subtle textures and detail against more visible noise. Ultimately only you can make the choice as to which is more important, but the Crispening level control gives you that choice.

EX1 and EX3 Picture Profiles.

These are the picture profiles that I am currently tending to favour for the EX1, EX1R and EX3. Please remember that picture profiles are entirely subjective. These settings work for me, that doesn’t mean they are perfect or for everyone. I like the images the cameras produce when I use these profiles. Please feel free to adapt them or modify them any way you choose. They work on any of the current EX cameras.

Vivid – Designed to help match the EX to a PDW-700. Gives vivid colours with a small shift away from yellow.

Matrix – Cinema, Matrix Level +60

R-G +8,  R-B +10,  G-R 0,  G-B +15,  B-R +5,  B-G +6

Detail Level -10 Frequency +20, Crispening -40 (if using gain use crispening +14)

Gamma Cinegamma 1

Black level -3, Black Gamma -35

Low Key Saturation -10

Natural C4 – Designed to give a neutral, natural looking image.

Matrix – Cinema, Matrix Level +35

Detail level -7, Frequency +30, Crispening -40 (if using gain use crispening +20)

Black Level -3, Low key Saturation -15

AC Punch – Gives a very high contrast, bold look.

Matric – Cinema, level +40

Gamma Standard 2, Knee level 80, Slope 0

R-G 0,  R-B +1,  G-R +12,  G-B +2,  B-R +11,  B-G 0

Detail Level -10, Frequency +30, Crispening -45

Black Level -4, Black Gamma -20.

AC Good to Grade – a general purpose setup to give good grading possibilities.

Matrix – Cinema, Level +25

Gamma Cinegamma 1 (Do not use -3db gain)

Detail Level -7, Frequency +45, Crispening -45 (use +35 if using gain)

Black Level -3.

AC-SD Camera look. To mimic an older SD camcorder based on a DSR400, good for HD to SD conversion.

Matrix – Cinema, Level +15

Detail Level +20, Detail Frequency -35, White Limit +35, Black limit +45

Knee, Manual, Level 90, Slope 0.

Gamma Standard 2, Gamma Level +5

Black Gamma -10

Black Level -10

 

 

Enjoy! Any feedback or suggestions welcome. Let me know of any profiles that you come up with that may be of interest to others.

 

S-Log. A Further In Depth Look.

Well I posted here a few days ago about how Data was distributed across the S-Log curve. David williams (thanks David) questioned some of the things in my post raising some valid questions over it’s accuracy, so I withdrew the post in order to review it further. While the general principles within the post were correct (to the best of my knowledge and research) and I stand by them, some of the numbers given were not quite right and the data/exposure chart was not quite right.

Before going further lets consider the differences between the a video sensor works and the way our eyes work. A video sensor is a linear device while our own visual system is a logarithmic system. Imagine you are in a room with 8  light fittings, each one with the same power and light output. You start with one lamp on, then turn on another. When you turn on the second lamp the room does not appear to get twice as bright even though the amount of light in the room has actually doubled. Now with two lamps on what happens when you turn on a third? Well you wouldn’t actually notice much of a change. To see a significant change you would need to turn on 2 more lamps. Now with 4 lamps on to see a significant difference you would need to turn on a further 4 lamps. Only adding one or two would make little visual difference. This is because our visual system is essentially a logarithmic system.

Now lets think about F-Stops. An f stop (or T-stop) is a doubling or halving of exposure. So again this is a logarithmic system. If with one light bulb your scene is one stop then to increase the scene brightness by one stop you must double the amount of light, so you would add another light bulb. Now to increase the scene brightness by a further stop you would have to take your existing two light bulbs and double it again to 4 light bulbs, and so on… 2, 4, 8, 16, 32, 64….

Now going back to a video sensor, take a look at the illustrative graph below. The horizontal scale is the number of lightbulbs in our hypothetical room and the vertical scale is the video output from an imaginary video sensor in percent. Please note that I am trying to illustrate a point, the numbers etc are not accurate, I’m just trying to explain something that is perhaps miss-understood by many, simply because it is difficult to understand or poorly explained elsewhere. The important thing to note is that the plotted blue line is a straight line, not a curve because the sensor is a linear device.

Linear Output from Camera Sensor

 

Now look at this very similar chart. The only difference now is that I have added an f-stop scale to the horizontal axis. Remember that one f-stop is a doubling of the amount of light, not simply one more lightbulb. I have also changed the vertical scale to data bits. To keep things simple I’m going to use something close 10 bit recording which actually has 956 data bits or steps (bits 64 to 1019 out of 1024 bits), but lets just round that up to 1000 data bits to keep life simple for this example.

Sensor data and f-stops

So we can see that this imaginary  video sensor uses bits 0-50 for the first stop, 50-100 for the second stop, 100-200 for the third stop, 200-400 for the fourth and 400-800 for the fifth. So it is easy to see that huge amounts of data are required to record each stop of over exposure. The brighter the image the more data that is required. Clearly if you want to record a wide dynamic range using a linear system you need massive numbers of data bits for the highlights, while the all important mid tones and shadow areas have relatively little data allocated to them. This is obviously not a desirable situation with current data limited recording systems, you really want to have sufficient data allocated to your mid-tones so that in post production you can grade them satisfactorily.

Now look what happens if we allocate the same amount of data to each stop of exposure. The green line is what you get if, in our imaginary camera we use 200 data bits to record each of our 5 stops of dynamic range. Does the shape of this curve look familiar to anyone? The important note here is that compared to the sensors linear output (the blue line) as the image brightness increases less and less data is being used to record the highlights. This mimics the way we see the world and helps ensure that in the mid ranges where skin tones normally reside there is lots of data to play with in post. Our visual system is most acute in the mid range. that’s because some of the most important things that we see are natural tones, plants, fauna and people. We tend to pay much less attention to highlights as these are rarely of interest to us. Because of this we can afford to reduce the amount of information in video highlights without the end user really noticing. This technique is used by most video cameras when the knee kicks in and compresses highlights. It’s also used by extended gamma curves such as cinegamma’s and hypergamma’s.

Log Curve, 200 bits for each stop.

Anyone that’s seen a hypergamma curve or cinegamma curve plot will have seen a similar shape of curve. Hypergammas and Cinegammas also use less and less data to record highlights (compared to a linear response) and in many ways achieve a similar improvement in the captured dynamic range.

Sony Hypergamma Curves

Hypergammas are not the same as S-Log however. Hypergammas are designed to be useable without grading, even if it’s not ideal. Because of this they stay close to standard gammas in the mid range and it’s only really the highlights that are compressed, this also helps with grading if recording using only an 8 bit codec as the amount of pushing and pulling required to get a natural image is less extreme. However because the Hypergammas allocate more data in the 60 to 90 percent exposure range to stay close to standard gamma the highlights have to be more highly compressed than S-Log so there is less highlight data to work with than with S-Log.  If we look at the plot below which now includes an approximate S-Log curve (pink line) you can see that log recording has a much larger difference from a standard gamma in the mid ranges, so heavy grading will be required to get a natural looking image.

Hypergammas and S-Log curves

Because of the amount of grading that will normally be done with S-Log, recording the output using a 10 bit recorder is all but essential.

When I wrote this article I spent a lot of time studying the Sony S-Log white paper and reading up on S-Log and gamma curves all over the place. One thing that I believe leads to some confusion is the way Sony presents the S-Log data curve in the document. The exposure is plotted against the data bits using stops as opposed to image brightness. This is a little confusing if you are used to seeing traditional plots of gamma curves like the ones I have presented above that plot output against percentage light input. It’s confusing as Sony forget that using stops as the horizontal scale means that the horizontal scale is a log scale and this makes the S-Log  “curve”  appear to be a near straight line.

I have not used S-Log on an F3 yet. It will be interesting to see how it compares to Hypergamma in the real world. I’m sure it will bring some advantages as it allows for an 800% exposure range. I welcome any comments or corrections to this article.