Tag Archives: knee

Log and Raw Don’t have highlight a highlight roll off.

This just keeps coming up over and over. Almost all log gamma curves and the majority of raw recording formats don’t have a highlight roll-off. Any roll off that you might see is probably in the LUT’s that you are using.

The whole point of log and raw is to capture as much information about the scene that you are shooing as you can. Log normally achieves this by recording every stop above middle grey with a constant amount of data, so even the very brightest stop has the same amount of recording data as the ones below it – there is no roll off.

In conventional limited range recordings such as Rec-709, hypergamma, cinegamma etc, highlight roll-offs work by reducing the contrast in the highlights to make the amount of data needed to record the very brightest stops much smaller than used for the rest of the image. This allows 2 or 3 stops to be squeezed into a very small recording range, keeping most of the recording data available for a nice bright high contrast image. The reduction in contrast in the extreme highlights helps hide any highlight handling problems and makes it appear as though the sensors clipping point is reach in a more pleasing soft manner.

But you don’t want this in a log or raw recording as it makes grading much harder as the footage will contain different contrast ranges, each needing it’s own grading adjustments. Also by reducing contrast in the highlights you are reducing the data. It would be very difficult to un-pick a highlight roll off and if you did want to expand the data back out you will get issues such as banding.

S-log-levels Log and Raw Don't have highlight a highlight roll off.
Chart showing S-Log2 and S-Log3 plotted against f-stops and code values. Note how little data there is for each of the darker stops, the best data is above middle grey and there is no highlight roll-off. Note that current sensor only go to +6 stops over middle grey so S-Log2 and S-Log record to different peak levels.

S-Log2 and S-Log3 like almost all log gammas have no highlight roll-off. The only roll off is from middle grey and down. So if you underexpose you will start to roll away the data in your scenes mid range and that’s not good. Expose for the mid range, this is the most important part of any image. If your highlights are a bit clipped don’t worry about this too much. In post production you can add a roll off in the grade that will make any clipped highlights roll away gently. Adding a bit of highlight diffusion in post will also nicely mask any clipped highlights and make them look natural.

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Auto Knee when shooting with Rec-709.

Like many cameras the Sony PXW-FS7, PMW-F5 and F55 use an automatic knee circuit to help the camera handle strong highlights or overexposure when shooting using standard gamma curves such as Rec-709 (STD gamma 5). On some ENG cameras there is a very similar function  called DCC (Dynamic Contrast Compensation) which is often selected via the Camera/Bars switch.

On the FS7, F5/F55 and many others the Auto Knee is on by default out of the factory. It can be turned on and off in the cameras paint settings. In most normal shooting situations, if you are correctly exposed the auto knee does a good job of bringing bright highlights down out of clipping.  The auto knee threshold is at around 90% brightness. Expose with objects brighter than 90% in your scene and the auto knee starts to kick in.

The correct exposure for white, such as a 90% reflectivity white card or white piece of paper in Rec-709 is 90%. Skin tones, plants, walls, roads and in fact most objects will normally be below white or below 90%. However direct light sources, such as the sky or direct reflections such as shiny car body work will be brighter than white. So the knee should only ever effect objects brighter than white if you are exposed correctly.  So for most situations it should not effect skin tones and the majority of the scene, just the bright highlights.

The auto knee detects highlight levels above 90% and tries to keep the highlight range below clipping by adding contrast compression to the highlights. The amount of compression depends on how strong the highlights are. As a result the auto knee effect will vary with exposure. If you have a scene with only a few highlights there will be some knee compression and it’s effect will only be seen above approx 90%. If you then open the aperture or have a lot of highlights the auto knee will increase the highlight compression to compensate. If the highlight range becomes very large then the knee will not only increase the amount of compression but also lower the knee point so more and more of the upper exposure range is effected by the knee. In extreme cases the knee point may get as low as 70-80% and this then starts to effect skin tones.

To prevent rapid fluctuations of the contrast in the highlight range the auto knee has a slight delay. This can result in a vicious circle where you open the iris a bit to help brighten the shot. The shot gets brighter. Then a couple of seconds later you look at the shot again and because the knee has now adjusted the highlights after it’s delay period it looks different to how it looked at the moment you made the initial adjustment. So you adjust again…. then the knee adjusts again and so on. Sometimes this lag can make it tricky to get your highlights to look exactly how you want.

Another common auto knee effect is to see the brighter parts of an entire image change as a result of a change in only a small part of the scene. A typical example would be an interview with a window in the background. As the highlight level in the bright window changes, perhaps as the sun comes and goes from behind passing clouds, the knee tries to compensate and all of the highlights in the scene go up and down in brightness whether they are over exposed or not. This looks very strange and can ruin an otherwise good looking shot.

If you are shooting in a studio against a white background the auto knee makes it impossible to get a brilliantly bright uniformly clipped white background. You increase your exposure to make the white background extra bright and because that white is now above 90% the auto knee treats it as a highlight and tries to control it’s brightness. The more you open the aperture the more the knee pulls down the white background, it never reaches clipping. Eventually you get to the point where the knee starts to effect the skin tones but your white backdrop still isn’t clipped. The image doesn’t look great.

In these cases the best thing to do is to turn off the Auto Knee. If you go into the paint settings you will find the knee settings. In most cases leave the knee on (except perhaps for the white studio example), but turn OFF the auto knee function. The fixed level knee will still give you a good highlight range but eliminate the pumping or other variable knee effects. Note that the knee options have no effect if using a Hypergamma or log. They only come into paly with standard gamma.

When should I use a Cinegamma or Hypergamma?

Cinegammas are designed to be graded. The shape of the curve with steadily increasing compression from around 65-70% upwards tends to lead to a flat looking image, but maximises the cameras latitude (although similar can be achieved with a standard gamma and careful knee setting). The beauty of the cinegammas is that the gentle onset of the highlight compression means that grading will be able to extract a more natural image from the highlights. Note than Cinegamma 2 is broadcast safe and has slightly reduced recording range than CG 1,3 and 4.

Standard gammas will give a more natural looking picture right up to the point where the knee kicks in. From there up the signal is heavily compressed, so trying to extract subtle textures from highlights in post is difficult. The issue with standard gammas and the knee is that the image is either heavily compressed or not, there’s no middle ground.

In a perfect world you would control your lighting (turning down the sun if necessary ;-o) so that you could use standard gamma 3 (ITU 709 standard HD gamma) with no knee. Everything would be linear and nothing blown out. This would equate to a roughly 7 stop range. This nice linear signal would grade very well and give you a fantastic result. Careful use of graduated filters or studio lighting might still allow you to do this, but the real world is rarely restricted to a 7 stop brightness range. So we must use the knee or Cinegamma to prevent our highlights from looking ugly.

If you are committed to a workflow that will include grading, then Cinegammas are best. If you use them be very careful with your exposure, you don’t want to overexpose, especially where faces are involved. getting the exposure just right with cinegammas is harder than with standard gammas. If anything err on the side of caution and come down 1/2 a stop.

If your workflow might not include grading then stick to the standard gammas. They are a little more tolerant of slight over exposure because skin and foliage won’t get compressed until it gets up to the 80% mark (depending on your knee setting). Plus the image looks nicer straight out of the camera as the cameras gamma should be a close match to the monitors gamma.

Brewing up a Scene File: Gamma and Knee

Brewing up a Scene File: Gamma and Knee

Before anyone complains that I have missed stuff out or that some technical detail is not quite right, one of the things I’m trying to do here is simplify the hows and why’s to try and make it easier for the less technical people out there. Lets face it this is an art form, not a science (well actually a bit of both really).
So what is a gamma curve anyway? Well the good old fashioned cathode ray tube television was a very non-linear device. You put 1 unit of power in and get one unit of light out. You put 2 units in and get 1.5 units out, put 3 in and get 2 out… and so on. So in order to get a natural picture the output of the camera also has to be modified to compensate for this. This compensation is the gamma curve, an artificial modification of the output signal from the camera to make it match TV’s and monitors around the world. See Wikipedia for a fuller explaination:   http://en.wikipedia.org/wiki/Gamma_correction
So, all video cameras will have a gamma curve, whether you can adjust it or not is another matter. Certainly most pro level cameras allow you some form of gamma adjustment.
The PMW-350 has 6 standard gamma curves, these are all pretty similar, they have to be otherwise the pictures wouldn’t look right, but small changes in the curve effect the relationship between dark and bright parts of the pictures. Todays modern cameras have a far greater dynamic range (range of dark to bright) than older cameras. This means that the full dynamic range of the sensor no longer fits within the gamma curves used for TV’s and monitors. In broadcast television any signal that goes over 100% gets clipped off and is discarded, so the cameras entire brightness range has to be squeezed into 0 to 100%. The PMW-350 sensors are capable of far more than this (at least 600%) so what can you do?
The older and simpler solution is called the “Knee”. The knee works because in most cases the brightest parts of a scene contains little detail and is generally ignored by our brains. We humans tend to focus on mid-tone faces, animals and plants rather than the bright sky. Because of this you can compress the highlights (bright) parts of the picture quite heavily without it looking hugely un-natural (most of the time at least). What the knee does is takes a standard gamma curve and up near it’s top, bends it over. This has the effect of compressing the brighter parts of the image, squashing a broad range of highlights (clouds for example) into a narrow range of brightness. While this works fairly well, it does tend to look rather “electronic” as the picture is either natural (below the knee) or compressed (above the knee).
The answer to this electronic video look is to replace the hard knee with gentle bend to the gamma curve. This bend starts some way down the gamma curve, very gentle at first but getting harder and harder as you go up the gamma curve. This has the effect of compressing the image gently at first with the compression getting stronger and stronger as you go up the curve. This looks a lot more natural than a hard knee and is far closer to the way film handles highlights. The downside is that because the compression starts earlier a wider tonal range is compressed. This makes the pictures look flat and uninteresting. You have to watch exposure on faces as these can creep into the compressed part of the curve. The plus point is that it’s possible to squeeze large amounts of latitude into the 100% video range. This video can then be worked on in post production by the editor or colorist who can pull out the tonal range that best suits the production.
These compressed gamma curves are given different names on different products. Panasonic call them “Film Rec”, on the EX1 they are “Cinegammas” on the PMW-350 they are “Hypergammas”. The 350 has four Hypergammas. The first is 3250. this takes a brightness range the equivalent to 325% and compresses it down to 100%. HG 4600 takes 460% and squeezes that down to 100%. Both of these Hypergammas are “broadcast safe” and the recordings made with them can be broadcast straight from the camera without any issues. The next Hypergamma is 3259. This takes a 325% range and squeezes this down to a 109% range, likewise 4609 takes 460% down to 109%. But why 109%? well the extra 9% gives you almost 10% more data to work with in post production compared to broadcast safe 100%. It also gives you the peak white level you need for display on the internet. Of course if you are doing a broadcast show you will need to ensure that the video levels in the finished programme don’t exceed 100%.
My preferred gamma is Hypergamma 4 (4609) as this gives the maximum dynamic range and gives a natural look, however the pictures can look a little flat so if I’m going direct from the camera to finished video without grading I use either a standard gamma or use the Black Gamma function to modify the curve. I’ll explain the Black Gamma in my next post.
There are 6 standard gammas to choose from. I like to stick with gamma 5 which is the ITU-709 HD standard gamma. To increase the dynamic range I use the Knee. The default knee point setting is 90, this is a reasonable setting, but if your shooting with clipping set to 100% you are not getting all the cameras latitude (the Knee at 90 works very well with clipping at 108%). Lowering the knee down to 83 gives you almost another stop of latitude, but you have to be careful as skin tones and faces can creep up towards 83%. It’s very noticeable if skin becomes compressed so you need to watch your exposure. This is also true of the Hypergammas and with them you may need to underexpose faces very slightly. The other option is to set the knee point to 88 and then also adjust the knee slope. The slope is the compression amount. A positive value is more compressed, negative less compressed. With the knee at 88 and slope set to +20 you get good latitude, albeit with quite highly compressed highlights.
If you want to play with the gammas and knee and see how they work one method you can use is to use a paint package on your PC (such as photoshop) to create a full screen left to right graduated image going from Black to white. Then shoot this with the camera (slightly out of focus) while making adjustments to the curves or knee and record the results along with a vocal description of each setting. Import the clips into your favorite editing package and use the waveform monitor or scopes you should be able to see a reasonable representation of the shape of the gamma curve and knee.
So my Gamma Choices are:
For material that will be post produced: Hypergamma 4609 (HG4)
For material that will be used straight from the camera: Standard Gamma 5 Knee at 90 with clip at 108% for non broadcast or Knee at 88 with slope +20 with white clip at 100% for direct to broadcast.

Brewing up a Scene File: Gamma and Knee

Before anyone complains that I have missed stuff out or that some technical detail is not quite right, one of the things I’m trying to do here is simplify the hows and why’s to try and make it easier for the less technical people out there. Lets face it this is an art form, not a science (well actually a bit of both really).

So what is a gamma curve anyway? Well the good old fashioned cathode ray tube television was a very non-linear device. You put 1 unit of power in and get one unit of light out. You put 2 units in and get 1.5 units out, put 3 in and get 2 out… and so on. So in order to get a natural picture the output of the camera also has to be modified to compensate for this. This compensation is the gamma curve, an artificial modification of the output signal from the camera to make it match TV’s and monitors around the world. See Wikipedia for a fuller explaination:   http://en.wikipedia.org/wiki/Gamma_correction

So, all video cameras will have a gamma curve, whether you can adjust it or not is another matter. Certainly most pro level cameras allow you some form of gamma adjustment.

The PMW-350 has 6 standard gamma curves, these are all pretty similar, they have to be otherwise the pictures wouldn’t look right, but small changes in the curve effect the relationship between dark and bright parts of the pictures. Todays modern cameras have a far greater dynamic range (range of dark to bright) than older cameras. This means that the full dynamic range of the sensor no longer fits within the gamma curves used for TV’s and monitors. In broadcast television any signal that goes over 100% gets clipped off and is discarded, so the cameras entire brightness range has to be squeezed into 0 to 100%. The PMW-350 sensors are capable of far more than this (at least 600%) so what can you do?

The older and simpler solution is called the “Knee”. The knee works because in most cases the brightest parts of a scene contains little detail and is generally ignored by our brains. We humans tend to focus on mid-tone faces, animals and plants rather than the bright sky. Because of this you can compress the highlights (bright) parts of the picture quite heavily without it looking hugely un-natural (most of the time at least). What the knee does is takes a standard gamma curve and up near it’s top, bends it over. This has the effect of compressing the brighter parts of the image, squashing a broad range of highlights (clouds for example) into a narrow range of brightness. While this works fairly well, it does tend to look rather “electronic” as the picture is either natural (below the knee) or compressed (above the knee).

The answer to this electronic video look is to replace the hard knee with gentle bend to the gamma curve. This bend starts some way down the gamma curve, very gentle at first but getting harder and harder as you go up the gamma curve. This has the effect of compressing the image gently at first with the compression getting stronger and stronger as you go up the curve. This looks a lot more natural than a hard knee and is far closer to the way film handles highlights. The downside is that because the compression starts earlier a wider tonal range is compressed. This makes the pictures look flat and uninteresting. You have to watch exposure on faces as these can creep into the compressed part of the curve. The plus point is that it’s possible to squeeze large amounts of latitude into the 100% video range. This video can then be worked on in post production by the editor or colorist who can pull out the tonal range that best suits the production.

These compressed gamma curves are given different names on different products. Panasonic call them “Film Rec”, on the EX1 they are “Cinegammas” on the PMW-350 they are “Hypergammas”. The 350 has four Hypergammas. The first is 3250. this takes a brightness range the equivalent to 325% and compresses it down to 100%. HG 4600 takes 460% and squeezes that down to 100%. Both of these Hypergammas are “broadcast safe” and the recordings made with them can be broadcast straight from the camera without any issues. The next Hypergamma is 3259. This takes a 325% range and squeezes this down to a 109% range, likewise 4609 takes 460% down to 109%. But why 109%? well the extra 9% gives you almost 10% more data to work with in post production compared to broadcast safe 100%. It also gives you the peak white level you need for display on the internet. Of course if you are doing a broadcast show you will need to ensure that the video levels in the finished programme don’t exceed 100%.

My preferred gamma is Hypergamma 4 (4609) as this gives the maximum dynamic range and gives a natural look, however the pictures can look a little flat so if I’m going direct from the camera to finished video without grading I use either a standard gamma or use the Black Gamma function to modify the curve. I’ll explain the Black Gamma in my next post.

There are 6 standard gammas to choose from. I like to stick with gamma 5 which is the ITU-709 HD standard gamma. To increase the dynamic range I use the Knee. The default knee point setting is 90, this is a reasonable setting, but if your shooting with clipping set to 100% you are not getting all the cameras latitude (the Knee at 90 works very well with clipping at 108%). Lowering the knee down to 83 gives you almost another stop of latitude, but you have to be careful as skin tones and faces can creep up towards 83%. It’s very noticeable if skin becomes compressed so you need to watch your exposure. This is also true of the Hypergammas and with them you may need to underexpose faces very slightly. The other option is to set the knee point to 88 and then also adjust the knee slope. The slope is the compression amount. A positive value is more compressed, negative less compressed. With the knee at 88 and slope set to +20 you get good latitude, albeit with quite highly compressed highlights.

If you want to play with the gammas and knee and see how they work one method you can use is to use a paint package on your PC (such as photoshop) to create a full screen left to right graduated image going from Black to white. Then shoot this with the camera (slightly out of focus) while making adjustments to the curves or knee and record the results along with a vocal description of each setting. Import the clips into your favorite editing package and use the waveform monitor or scopes you should be able to see a reasonable representation of the shape of the gamma curve and knee.

So my Gamma Choices are:

For material that will be post produced: Hypergamma 4609 (HG4)

For material that will be used straight from the camera: Standard Gamma 5 Knee at 90 with clip at 108% for non broadcast or Knee at 88 with slope +20 with white clip at 100% for direct to broadcast.