Back Plates and filming for CGI

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Mike Fox BSC

With blue and green screen featuring as largely as it does on any Cinematographers horizon, it's gratifying that aside from complex motion control shots, it's all become a little more forgiving. Some technical ground rules do remain, good separation between subject and background in order to prevent spill and where possible, an even spread of light over the backing surface is still desirable, but the precise exposure differentials necessary for successful travelling matte photography - the forrunner of blue screen are not as critical.

I do have a very particular view though when it comes to the aesthetics. Leaving aside the obvious pitfalls - the subjects clothing if shooting against blue and hair colour if against green, where possible try and shoot the backgound plates as well as the foreground. If that's not an option at least talk to the person commissioned to do so, taking care to pass on, 'essential' information - focal length, exposure, the direction colour and quality of light and most importantly, point of focus - the human eye does not see sharp focus from five feet to infinity, neither should the camera.

It's a very unnerving thing to shoot soft backgrounds but if the subject has been captured in a midshot at T.2 that is what they would be! I have in my time had to claw a keyable background in circumstances that a few years ago would have been impossible - an overhead tracking shot of a horse and rider galloping on Astroturf successfully translated to the same horse and rider flying over a mountain range.

The availablity of highly reflective front projection material lit along the camera's axis by a ring of green or blue LEDs enables contact between subject and background - The sky's the limit and you can sometimes use that....that is when it's blue.

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Mike Fox is a highly respected and experienced cameraman and Director of Photography and a member of the British Society of Cinematographers
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