Gamma refers to the transfer curve relationship between the video level and the scene brightness. For obscure technical reasons it was decided right from square one of TV broadcasting to have a non linear logarithmic relationship such that the video voltage from the camera sensor was processed to give a continually decreasing slope. This relationship is reversed in the TV receiver so that the overall relationship of scene to screen is basically linear. The default gamma value for the camera is 0.45, that is to say the Vout is proportional to the Vin to the power 0.45. The inverse needed in the TV is 2.2. The traditional symbol amongst mathematicians for such an operator is a Greek letter Gamma, hence the name.
This all works fine for the limited controlled scene contrast in a studio. In real light, the ‘dynamic range’ or contrast ratio is too large for video to cope. One solution to help this is to reduce the contrast in the highlights, by using a different gamma value. In practice the point where the two values change is called the ‘knee point’ and the reduced contrast zone has a gamma known as the ‘knee slope’. The ‘knee’ parameters can either be fixed by values set in the set-up menu or can be controlled dynamically by the picture highlights. Sony calls this DCC (Dynamic Contrast Control) whilst Panasonic calls it ‘Auto Knee’.
A more radical solution is to resign all picture contrast control to post production and to set the camera to capture as much of the scene content as possible. (This is similar to the ‘Raw’ feature in digital still cameras.) Sony offer some suggestions to this end called ‘HyperGamma’ and users can also load their own curves called ‘UserGamma’. Using these values will result in pictures with apparent low contrast when viewed on a normal video display, but this can be overcome on location by using an inverse gamma function commonly provided by a LUT or CLUT (Color LookUp Table).
Operators will sometimes lower the black level of the signal to give an apparent increase in image contrast and colour saturation. This will also cause clipping of the darkest values and hence permanent loss of shadow detail. Cameras often now have a function called ‘Black Gamma’ which allows non permanent manipulation of the signal to get a similar effect.
