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.