Tag Archives: gamma

cinematography, Technology

Choosing the right gamma curve.

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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.

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To shoot flat or not to shoot flat?

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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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S-Log, Latitude, Dynamic Range and EI S-log. Or how to modify your exposure range with EI S-Log

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The big issue most people have when working with log and exposing mid grey at 38 is that when you look at it on a standard monitor without any lookup tables it looks underexposed. The assumption therefore is that it is underexposed or in some way too dark to ever look right, because that’s what people used to working with conventional gammas have become programmed to believe over many years from their experience with conventional gammas.

So, for confidence you add a lookup table which converts the log to a Rec-709 type gamma and now the image looks brighter, but as it now has to fit within Rec-709 space we have lost either some of our high end or low end so we are no longer seeing the full range of the captured image so highlights may be blown out or blacks may be crushed.
It’s important for people to understand the concept of gamma and colour space and how the only way to truly see what a camera (any camera) is capturing is to use a monitor that has the same gamma and colour space. Generally speaking lookup tables don’t help as they will be taking a signal with a large range and manipulating it to fit in a small range and when you do that, something has to be discarded. If you were to take an F3 set to S-log and expose mid grey at 38 and show that on one of the nice new Sony E170 series monitors that have S-log gamma and place that next to another F3 with Rec-709 shooting mig grey at 45% and a similar but conventional 709 monitor the lower and mid range exposures would be near identical and the S-log images would not look under exposed or flat. The S-log images however would show an extra 2 stops of dynamic range.

Furthermore it has to be remembered that log is log, it is not linear. Because of its non linear nature, less and less brightness information is getting recorded as you go up the brightness range. As our own visual system is tuned to be most accute in the mid ranges this is normally fine provide you expose correctly putting mid tones in the more linear, lower parts of the S-log curve. Start putting faces to high up the S-log curve and it gets progressively harder to get a natural look after grading. This is where I think a lot of people new to log stumble. They don’t have the confidence to expose faces at what looks like a couple of stops under where they would with a standard gamma, so they start bringing up the exposure closer to where they would with standard gamma and then have a really hard time getting faces to look natural in the grade. Remember that the nominal S-Log value for white is 68 IRE. Part of the reason for this is that above about 70 IRE the amount of compression being applied by log is getting pretty extreme. While there is some wriggle room to push your exposure above or below the nominal mid grey at 38 it’s not as big as you might expect, especially dealing with natural tones and overexposure.

If you do want to shift your middle grey point this is where the EI S-log function and a light meter comes into it’s own, it’s what it’s designed for.

First something to understand about conventional camera gain, dynamic range and latitude. The latitude and sensitivity of the F3 is governed by the latitude and sensitivity of the sensor, which is a little under 13 stops. Different amounts of gain or different ISO’s don’t alter the sensors latitude, nor do they alter the actual sensitivity, only the amount of signal amplification. Increasing the camera gain will reduce the cameras output dynamic range as something that is 100 IRE at 800 ISO would go into clipping if the actual camera gain was increased by 6db (taking the ISO to 1600) but the darkest object the camera can actually detect remains the same. Dark objects may appear brighter, but there is still a finite limit to how dark an object the camera can actually see and this is governed by the sensor and the sensors noise floor.

EI (Exposure Index) shooting works differently, whether it’s with the F3, F65, Red or Alexa. Let’s consider how it works with the PMW-F3. In EI S-Log mode the camera always actually outputs at 800 ISO from the A/B outputs. It is assumed that if your working with S-Log you will be recording using an external 10 bit recorder connected to the A/B outputs. 422 is OK, but you really, really need 10 bit for EI S-Log. At 800 ISO you have 6.5 stops of over exposure and 6.5 under when you shoot mid grey at 38 or expose conventionally with a light meter.
Now what happens when you set the camera to EI 1600? Understand that the camera will still output at 800 ISO over the A/B outputs to your external recorder, but also note that 6db gain (1 stop) is added to the monitor output and what you see on the LCD screen, so the monitor out and LCD image get brighter. As the cameras metering systems (zebras, spot meter, histogram) measure the signal on the monitor side these are also now offset by +6db or + 1 stop.
As the camera is set to EI 1600 we set our light meter to 1600 ISO. If we make no change to our lighting the light meter would tell us to stop down by one stop, compared to our original 800 ISO exposure.
Alternately, looking at the camera, when you switch on EI 1600 the picture gets brighter, your mid grey card would also become brighter by one stop, so If we use the cameras spot meter to expose our grey card at 38 again we would need to stop down the iris by one stop to return the grey card to 38 IRE (for the same light levels as we used for 800). So either way, whether exposing with a light meter or exposing using the cameras built in metering, when you go from EI 800 to EI 1600 for the correct exposure (under the same lighting) you would stop down the iris by one stop.
Hope those new to this are still with me at this point!
Because the cameras A/B output is still operating at 800 ISO and you have stopped down by one stop as that what the light meter or camera metering told you to do because they are operating at EI 1600, the A/B output gets darker by one stop. Because you have shifted the actual recorded output down by one stop you have altered you exposure range from the original +/- 6.5 stops to + 7.5 stops, -5.5 stops. So you can see that when working at EI 1600 the dynamic range now becomes + 7.5 stops and -5.5 stops. Go to EI 3200 and the dynamic range becomes +8.5 stops and -4.5 stops.
So EI S-log gives you a great way of shifting your dynamic range centre while giving you consistent looking exposure and a reasonable approximation of how your noise levels are changing as you shift your exposure up and down within the cameras dynamic range.
EI S-Log doesn’t go below 800 because shifting the dynamic range up the exposure range is less beneficial. Lets pretend you have an EI 400 setting. If you did use it, you would be opening up the iris by one stop, so your range becomes +5.5 and -7.5 stops compared to your mid grey or light metered exposure. So you are working with reduced headroom and you are pushing your mid range up into the more highly compressed part of the curve which is less desirable. I believe this is why the option is not given on the F3.

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REC-709 – A true REC-709 camera should only have 5 stops of dynamic range!

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In theory if two cameras are both set to REC-709 then the dynamic range should be identical as both cameras responses will be limited to the REC-709 specifications. The original REC-709 specification only allows for 5 stops of dynamic range from 0 to 100%. The addition of superwhites at 109% gives a little more and then the knee on top gives a bit more again. The whole point behind REC-709 is that when a REC-709 camera captures something of a specific brightness, then when that image is shown on a TV or monitor, the apparent brightness will be exactly the same. As REC-709 is based on legacy television standards that go back to the very beginning of television broadcasting, it only allows for the dynamic range of older television sets. If a camera deviates from REC-709 then there will be a miss-match between the image the camera records and the image the viewer sees.

So, if we have two cameras with sensors capable of a dynamic range greater than the REC-709 specifications and then we restrict them to REC-709, they should exhibit identical dynamic range. Just like two cars fitted with speed limiters, they would both have identical maximum speeds.  In fact the REC-709 specification is actually a viewing and display specification, not a camera specification and in reality the REC-709 gamma curves in most cameras don’t correspond exactly to the true 709 monitor specs as everyone would be up in arms if the camera only had a 5 stop range.

With almost any reasonably up to date cameras, anyone comparing a couple of cameras dynamic range with REC-709 and saying A has more dynamic range than B is a bit of a misnomer. Yes there will be differences in the way the gamma curve has been interpreted by the manufacturer or the way the knee works and perhaps camera A will give a more pleasing picture than B, but this is unlikely to actually be due to the dynamic range of the camera, just different ways the manufacturer has chosen to interpret the restrictions imposed by REC-709.

In my opinion, the only times you would want to use 709 is when you are shooting direct for broadcast where there won’t be time for grading or if you have enough control over your lighting to stay within a 6 stop range and that 6 stop range gives you the look you want.

Otherwise, I would use one of the extended range gammas included in most cameras these days, log, cinegamma, hypergamma etc, as these extended range gammas don’t try to comply to the REC-709 specifications they are not restricted to the legacy dynamic range imposed on us by REC-709. As a result they can easily accommodate much greater dynamic ranges and get closer to utilising the camera sensors full range, but at the expense of requiring some grading to restore contrast.

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Canon C-Log on the C300 compared to S-Log.

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First let me say that as yet I have not used C-Log in anger, only seen it at a couple of hands on demo events and in downloaded clips.

From what I’ve seen C-Log and S-Log are two quite different things. S-Log on the F3 is a true Log curve where each stop of exposure is recorded using roughly the same amount of data and the available dynamic range is about 13.5 stops. It is inevitable that when you use a true log curve like this and play it back on an uncorrected Rec-709 (standard HD gamma) monitor that it will look very flat and very washed out. This is a result of the extreme gamma miss-match across the entire recording range. If you had a monitor that could display 13.5 stops (most only manage 7) and the monitor had a built in Log curve then the pictures would look normal.

What has too be considered is that S-Log is designed to be used with 10 bit recording where each stop gets roughly 70 data bits ( this roughly means 70 shades of grey for each stop).

Now lets consider the Canon C300. It has no 10 bit out, it’s only 8 bit. Assuming Canon’s sensor can handle 13.5 stops then using 8 bit would result in only 17 bits per stop and this really is not sufficient, especially for critical areas of the image like faces and skin tones. A standard gamma, without knee, like Rec-709 will typically have a 7 stop range, this is a deliberate design decision as this yields around 34 bits per stop. As we know already if you try to do a hard grade on 8 bit material you can run in to issues with banding, posterisation and stair stepping, so reducing the bits per stop still further (for example by cramming 13.5 stops into 8 bits) is not really desirable as while it can improve dynamic range, it will introduce a whole host of other issues.

Now for some years camera sensors have been able to exceed 7 stops of dynamic range. To get around the gamma limitation of 7 stops, most good quality cameras use something called the knee. The knee takes the top 15 to 20% of the recording range to record as much as 4 to 5 stops of highlights. So in the first 0 to 80% range you have 6 stops, plus another 4 to 5 stops in the last 20%, so the overall dynamic range of the camera will be 10 to 11 stops.

How can this work and still look natural? Well our own visual system is tuned to concentrate on the mid range, faces, foliage etc and to a large degree highlights are ignored. So recording in this way, compressing the highlights mimics they way we see the world, so doesn’t actually look terribly un-natural. OK, OK, I can hear you all screaming… yes it is un-natural, it looks like video! It looks like video because the knee is either on or off, the image is either compressed very heavily or not at all, there is no middle ground. It’s also hard to grade as mid tones and highlights have different amounts of squashing which can lead to some strange results.

So the knee is a step forward. It does work quite well for many applications as it preserves those 34 bits of data for the all important mid tones and as a result the pictures look normal, yet gives a reasonable amount of over exposure performance. Next came things like cine gammas and film style gammas.

These often share a very similar gamma curve to standard gammas for the first 60-70% of the recording range, so faces, skin, flora and fauna still have plenty of data allocated to them. Above 70% the image becomes compressed, but instead of the sudden onset of compression as with a knee, the compression starts very gently and gradually increases more and more until by the time you get close to 100% the compression is very strong indeed. This tends to look a lot more natural than gamma + knee, yet can still cope with a good over exposure range, but depending on the scene it can start to look a little flat as your overall captured range is biased towards highlights, so your captured image contains more bright range than low range so will possibly (but not always) look very slightly washed out. In my opinion, if shooting with cinegammas or similar you should really be grading your material for the best results.

Anyway, back to the Canon C300. From what I can tell, C-Log is an extension of the cinegamma type of gamma curve. It appears to have more in common with cinegammas than true S-log. It looks like the compression starts at around 60% and that there is a little more gain at the bottom of the curve to lift shadows a little. This earlier start to the compression will allow for a greater dynamic range but will mean fewer bits of data for skin tones etc. The raised lower end gain means you can afford to underexpose more if you need to. As the curve is not a full log curve it will look a lot more agreeable than S-Log on an uncorrected monitor, especially as the crucial mid tone area is largely unaffected by strong compression and thus a large gamma miss-match.

For the C300 this curve makes complete sense. It looks like a good match for the cameras 8 bit recording giving a decent dynamic range improvement, largely through highlight compression (spread over more recording range than a conventional knee or cinegamma), keeping mid tones reasonably intact and a little bit of shadow lift. Keeping the mid range fairly “normal” is a wise move that will still give good grading latitude without posterisation issues on mid range natural textures.

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More Codec and Gamma Tests.

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More Gemini, Samurai, AC-Log and S-Log sample frame grabs. See download box at bottom of post.
I had thought, when I first wrote this post that I had discovered a strange issue where the 444 RGB recordings from the Gemini had more dynamic range than 422 recordings. I didn’t think this was right, but it was what my NLE’s (FCP and CS5.5) were telling me. Anyway to cut a long story short, what was happening was that when I dropped the Gemini RGB files into the timeline the levels got mapped to legal levels, i.e. nothing over 100% while the YCbCr 422 clips go into the timeline at their original levels. The end result was that it appeared that the 422 clips were clipping before the 444 clips. Thanks to Waho for suggesting that it may be a conversion issue with the frame grabs, I was able to see that it was simply the way the NLE’s (both CS5.5 and FCP were behaving in the same way) were clipping off anything in the 422 clips above 100% both in the frame grabs and also on the monitor output. As the RGB files were all below 100% they were not clipped so appeared to have greater dynamic range.

Anyway….. below is a new set of frame grabs layered up in a single photoshop file showing how the various codecs and recorders and codecs perform. The levels in these have been normalised at 100% to avoid any dodgy clipping issues. I’ve included F3 Cinegamma 4, plus my AC-Log picture profile, plus Samurai ProRes, Gemini S-Log and F3 Internally recorded S-Log of a very extreme contrast situation. Use the link below to download the photoshop layers file. You’ll need to me a registered user to access the link.

[downloads_box title=”More codec test grabs.”]
Photoshop Layered Frame Grabs v3
[/downloads_box]

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

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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
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EX1 and EX3 Picture Profiles.

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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.

 

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More on S-Log and Gamma Curves

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A lot of the issues with any camera and the dynamic range it can record are not due to limitations of the cameras hardware but to retain compatibility with existing display technologies, in particular the good old fashioned TV set that has been around for half a century. The issue being that in order for all TV owners to see a picture that looks “natural” there has to be a common standard for the signal sent to the TV’s that will work with all sets from the very oldest to the most recent.

As even the most recent TV’s and monitors often struggle to display a contrast range greater than 7 stops there is no point in attempting to  feed them with more, Taking 12 stops and simply squashing it into 7 stops will create a disappointing, flat and dull looking image. So for productions where extensive grading is not taking place, it is not desirable to record information beyond that which the existing broadcast system can handle. This is why the vast majority of modern camcorders with the knee off and using a standard gamma curve all exhibit very similar dynamic ranges (7 to 8 stops typically), because the limitation is generally not that of the sensor, but that of the gamma curves used in broadcast television. By adding a bit of highlight compression through a cameras knee circuit we can stretch out the dynamic range a bit as our visual system is most acute to inaccuracies in the the mid ranges of an image where faces, people and natural subjects normally appear so we don’t tend to notice strong compression occurring in highlights such as the sky or reflections. A well designed knee circuit can help gain an extra 2 or 3 stops by compressing the hell out of highlights, but as most of us are probably aware it can create it’s own issues with the near complete loss of real detail in clouds and the sky as well as color saturation issues on skin highlights, this is gamma curve compression rearing it’s ugly head. Moving on, we come to cinegammas, hypergammas and other similar extended range gammas. One of the issues with a traditional aggressive knee circuit is that it is either on or off, compressing or not compressing, there is no middle ground and this makes grading problematic as it is all but impossible to extract any meaningful data from very highly compressed highlights. Cinegammas etc address this by slowly increasing the amount of compression used as image brightness increases. In addition the gamma curve compression starts much earlier, long before you get to what would traditionally be regarded as “highlights”. This slow and gentle onset of compression grades in a more pleasing manner than a conventional knee. If you don’t grade the added mid-to-highlight compression results in a picture that looks a little flat and lacks “punch”, but is not overly objectionable to view. There is however a limit to just how much data you can cram into a compressed codec or recording system. Cinegammas and Hypergammas are tailored to give optimum performance with existing 8 bit and 10 bit high compression systems and workflows so the design engineers chose to only record a range of about 11 stops as trying to extract more than this from systems essentially designed to only record 7 to 8 stops will lead to visible compression artefacts. Technologies have continued to advance and now it’s remarkably easy (compared to just a couple of years ago) to record 10 bits of 4:2:2 or 4:4:4 data without compression or with only minimal compression. By eliminating or at least significantly reducing the compression artefacts it’s now possible to extract more meaningful data from a compressed gamma curve than was possible previously. S-Log is in effect nothing more than a heavily modified gamma curve, taking cinegammas and hypergammas to the next level. S-Log needs 10 bit recording to work as the curve compression starts much lower in the curve, so when grading those crucial skin tones and natural objects will need to be un-compressed to look natural and 8 bits of data just would not give enough range. As the image brightness increases the amount of gamma curve compression is increased logarithmically. If you look at the data being recorded this means that the majority of the 10 bit data is allocated to shadow areas then mid tones with less and less data being used to record highlights.
Most modern cameras, not just the XDCAM’s simply ignore highlight information beyond what can be recorded, this results in the image getting clipped at a given point depending on the gamma curve being used. Interestingly using negative gain on a camcorder can act as a low end clip as very small brightness changes will be reduced by the negative gain, possibly to the point where they are no longer visible. This  normally results in a reduction in dynamic range (as well as noise). I suspect this is why the F3 has less noise using standard gammas because the sensor has excess dynamic range for theses curves and good sensitivity, so Sony can afford to set the arbitrary 0db point in negative space without impacting the recorded DR but giving a low noise floor benefit. For S-Log however it’s possible to record a greater dynamic range so 0db is returned to true zero and as a result the noise floor increases a little.
LUT’s are just a reverse gamma curve applied to the S-Log curve to restore the curve to one that approximates a standard gamma, normally REC-709. They are there for convenience to provide an approximation of what the finished image might look like. However applying an off the shelf LUT will impact the dynamic range as an assumption has to be made as to which parts of the image to keep and which to discard as we are back to squeezing 12 bits into 7 bits. As every project, possibly every shot will have differing requirements you would need an infinite number of LUT’s to be able to simply hit an “add LUT” button to restore your footage to something sensible. Instead it is more usual for the colorist or grader to generate their own curves to apply to the footage. Most NLE’s already have the filters to do this, it’s simply a case of using a curves filter or gamma curve correction to generate your own curves that can be applied to your clips in lieu of a LUT.

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PMW F3 Picture Profile Smorgasbord.

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I’ve been working some more on picture profiles for the PMW-F3, mainly matrix settings. You can download the full set by clicking here: ac-profiles. Download the zip file, unzip and place the “Sony” folder in the root of an SxS card or SD card in an adapter. Place the card in the camera and go into the “picture profiles” menu and select a picture profile and then “ppdata” and “recall” to load the data into your camera. This will overwrite any PP’s you already have.

Here’s the latest settings I have:

ALL use Detail level -17, Frequency +20, Aperture +25 unless otherwise stated.

AC Warm1: Warm look, less blue/yellow

Cinegamma 1, Black Gamma -25, Black Level -2.

Matrix: Standard, level +8, R-G +14, R-B +12, G-R +4, G-B +8, B-R +4, B-G -18

AC Cool1: Stark cool look, maybe day for night.

Cinegamma 1, Black Gamma -25, Black Level -2.

Matrix: Standard, level +22, R-G -44, R-B -24, G-R -34, G-B =28, B-R -7, B-G -69

AC Elec1:  Electronic, vivid look.

Gamma STD1, Black Gamma -20, black level -3, Detail Level -10, Frequency -40

Matrix Hi-Sat,

NAT1CG-1: Neutral Look, natural colors, less yellow/green.

Cinegamma 1, Black Level -2

Matrix FL-Light, Level +3, R-G +2, R-B +2, G-R +8, G-B +8, B-R -8, B-G -6

Note that for most of these I have used a cinegamma, that is because I would assume that post work will be done on the footage. If your not planning on doing any grading or post work you should consider using a standard gamma which will give a richer looking image or cinegamma 2 which is broadcast safe.