12 Tips For Getting a Better Green Screen Key

Since we're about a month away from the 2015 NAB Show, I thought it would be timely to look back at some of the useful information I came away with from last year's sessions.

In this post, I want to talk about green screen workflows. Those of you who have worked on green screen shoots before (and that's probably a majority of you) know how important it is to properly light your green screen and light and place your talent. Otherwise, the edges of your key will be blotchy and your talent will have a green glow.

I had the opportunity to attend Jeff Foster's class, in which he provided some extremely valuable information for setting up, lighting, and shooting green screens. By adhering to his tips, you can get a nice, clean key in post-production.

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How to Expose Properly

Proper exposure is crucial if you want to retain as much information in your shot as possible. If you happen to overexpose the frame and clip the highlights, that information is lost. No amount of correction and grading will bring back lost highlights. Conversely, more can be done to salvage lost shadow detail, but the trade-off is more noise, grain, and artifacting (depending on the camera and how crushed the shot is).

This is why using a waveform monitor is so important while on set. It reads luminance values from black (value=0) to white (value=100). Proper exposure means that everything within your frame falls above 0 and below 100. A good rule of thumb is to keep your midtones around 75. This is why, when I use zebras on my EVF, I keep them set to 75. Then I open up the iris enough to see a small amount of zebras on the forehead and/or cheekbones of my subject. That tells me that the midtones are hitting right around 75 IRE.

Another great way, besides using the zebras, to ensure proper exposure, is to use a middle gray card. Where middle gray should hit on your waveform monitor depends on the camera. The following information comes from Jem Schofield of The C47:

  • Canon (for the C100, C300, C500) recommends that middle gray hit around 33 IRE.
  • Arri recommends 38 IRE, but 40 if you are shooting LOG.
  • Middle gray should fall around 50 IRE if you are using the Standard picture profile on the 5D Mk III; 40 if you are using the Neutral profile.
  • Middle gray should fall around 40 IRE if you are using the Blackmagic Pocket Cinema Camera in film mode. 

Be sure to leave your tips and suggestions in the Comments below.