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HiDef Blue/Green Screen
Published : 22nd August 2003
Dear Fellow Cinematographers and Technicians,
I am going to shoot a project that is going to be all bluescreen/greenscreen.
I have some Ideas on how I would like to do this but want to get some feedback.
I am shooting HiDef and was wondering if the camera prefers Blue or Green screen and if the screen should be under key in the exposure or over key, assuming the key is not at a low t stop.
Any feedback is welcome.
Ed wrote :
>I am shooting HiDef and was wondering if the camera prefers Blue or >Green screen…
There are many variables with blue or green screen that need to be considered. They all need testing beforehand, as you know there is limited time while you are shooting.
* What is the compositing program you are using? (Ultimatte or Primatte, etc.) Some software do a little better than others with fine detail and general key pulls.
* What color of hair are on your key actors scalp? Blond actors on an slightly over-exposed green screen is harder to rescue than blue screen, especially with camera movement and action scenes.
Don't blow out the highlights with hair and try to keep the whole head in focus, if possible to avoid digital haircuts. An underexposed blue screen reveals its true noise at the bottom of the prism, and at -3db gain in a controlled environment, it can turn ugly stretched out noise into a normal looking "grain".
* Detail and color matrix set ups on your camera can be your compositors worst enemy. A camera matrix that looks good on a normal set can give you a false sense of evenness on the screen set monitor. One can always garbage matte the outside slop. Color correct later with more control and quality to reclaim what you are missing by shooting in a more raw set up. It is the edge quality for the key that is important, any edge detail can cause a bad effect, keep it smooth.
* Do you have a controlled environment or location work? Your at an extreme disadvantage without a waveform monitor on set. An uneven screen with variable densities will add time to the composite. Tight spaces create wraparound spill and tinted lens flare, flag off what you don't need. Green is easier to light in an exterior setting with even indirect daylight. Blue seems nicer in a moodier lighting setup and just to work around with your eyes.
* Screen exposure should really never be over key, unless there is no key, as in a dim moment or silhouette shot. Actually, once you have lit for the screen, you have your T-stop, (+/-), then your key light is built around that exposure setup. Lock it into your waveform monitor.
It is helpful when the clock is ticking past at hour ten. Think about your depth of field beforehand and what you will need focus wise to base your screen T-stop at. Have the director rehearse some potential blocking in a like space and you use a tape measure to make notes ahead of time. And if you want to use a polarising filter for the floor or highlight reduction, there goes some more T-stop.
Blue/Green screen lighting is not a dim subject, unless you never see the actors feet it seems.
Let me know - Good Luck,
Director of Photography
Los Angeles, CA