A question poped up today asking about how to expose S-Cinetone when shooting green screen.
The answer is really quite simple – no differently to how you would expose S-Cinetone anywhere else. But, having said that it is important to understand that S-Cinetone is a bit different to normal Rec-709 and this needs to be considered when shooting for chroma key or green screen.
S-Cinetone’s highlight roll off and shoulder starts much lower than most “normal” rec-709 curves. From around 73% the gamma curve changes and starts to compress the levels and reduce contrast. In the shdows there is a variable toe that increases contrast at lower brightness levels. The nominal “normal” brightness levels are also lower, all part of the contemporary film like look S-Cinetone is designed to give. A 90% reflectivity reference white card would be exposed at approx 83% instead of the more normal 90% (if you were using a light meter you should end up with a 90% white card at 83IRE). A white piece of paper will be a bit brighter than this as printer and copier paper etc is designed to look as bright as possible, typically printer paper comes out around 3 to 5% brighter than a proper white card.
The lower start to the highlight roll-off means that if you place skintones around 70% the brighter parts of a face will be affected by the rolloff and this will make them flatter. Expose skin tones at 60% and the face will be more contrasty and in my opinion look better. Although darker this would still be well with the “normal” exposure range for S-Cinetone so you will not have excessive noise and it will still key well.
S-Cinetone would be considered correctly exposed when a 90% white card is exposed between 78% and 88%. This is quite a wide window and is due to the way S-Cinetone is designed to give differening contrast levels simply by exposing a touch brighter or darker. The variable toe and shoulder mean that exposing brighter will make the image flatter and exposing darker more contrasty. Exposing as you would with normal Rec-709 levels with a white card at 90% will place skintones rather higher than “normal” and they will appear very flat. So either expose so a white card falls in the 78-88% window or use a calibrated monitor to observe how the skin tone look and be careful not to overexpose them.
Your greenscreen should be between 40IRE and 60IRE for a good clean key, I normally aim for 50IRE with S-Cinetone, but provided you don’t go below 40IRE or above 60IRE you should be good.