S-Log and contrast ratios. Why is Grey 38 and white 68?

My video camera operators are unfamiliar with contrast ratios, mid grey and white cards. With older restricted dynamic range video cameras you simply looked in the viewfinder and if the picture looked right, then it was right. Simples.

But now with extended range gammas, log curves and linear raw recording things are much more difficult as sometimes the image your looking at in the viewfinder is very different to the way the image will look after post production. This makes judging exposure harder. As a result you often need to resort to extra tools to help and one of the most common tools is the grey card and white card. A grey card should reflect 18% of the light that falls on it. A good white card will reflect 90% of the light falling on it. If you view a white, grey and black card side by side the 18% grey card will appear to your eye to be half way between the white and black card, so it’s know as “middle grey”.

The important thing to note is that we are not looking at finite values here. We are looking at ratios. The grey card reflects 18% of the light falling on the scene. The white card reflects 90% of the light, so the white card is always reflecting 500% more light than the grey card. It doesn’t matter how bright or dark the scene, the white card will always be 500% brighter than the grey card.

Another ratio is the f-stop. An exposure increase of one stop is a doubling of the light or 100% more. if you add another stop then that’s another doubling, so thats another 200% (total 300%), double it again and we add a further 400% (total 700%). From this we can deduce that if white is 500% brighter than mid grey, then white will be 2.5 stops brighter than mid grey, no matter how bright the total scene illumination. These are ratios, so the light level doesn’t matter, the ratio between the two remains constant.

This can be a difficult concept to grasp as it’s about ratios. Also remember we are looking at light reflected from cards with a know reflectance. We are not including direct light sources like a bright sky or a lamp that can be many stops brighter than the white card.

If you using S-Log then the normal exposure for middle grey is 38% of the way up the S-Log curve. Because the curve is not linear and compressed (squashed) a white card will be at 68% on the S-Log curve, not 90%. It doesn’t matter how bright or dark the scene that’s where the fixed reflection ratio of 500% or 2.5 stops between grey and white cause the levels to fall when you expose correctly using S-Log.