Post Production

Greenscreen Exposure

Exposure on the greenscreen

Photo by Flickr user Zoovroo

Let us assume that the camera choices are optimal, screen materials and lighting are ideal, and the foreground lighting matches the background lighting perfectly.

A common misconception is that backing brightness should be adjusted to match the level of foreground illumination. In fact, the optimum backing brightness depends only on the f-stop at which the scene is shot. Thus, normally lit day scenes and low-key night scenes require the same backing brightness if the appropriate f-stop is the same for both scenes. The goal is to achieve the same blue or green density on the negative, or at the sensor, in the backing area for every shot at any f-stop.

Imagine a plume of black smoke shot against a white background (Figure 1). It’s a perfect white: The measured brightness is the same in red, green, and blue records. The density of the smoke in the left-hand image ranges from dead black to just a whisper. What exposure of that white backing will capture the full range of transparencies of that smoke plume?  Obviously it’s the best compromise exposure that lands the white backing at the white point toward the top of the straight line portion of the H&D curve in film (a white-shirt white), or a level of 90% in video, and brings most dark values in the smoke up off the toe. If the backing was overexposed, the thin wisps would be pushed onto the shoulder and compressed (or clipped in video) and pinched out by lens flare. If the backing was underexposed (reproduced as a shade of gray), detail in the darkest areas would fall on the toe to be compressed or lost entirely. You could make up for underexposure by boosting the image contrast. As the right-hand image shows, this makes the backing white (clear) again, but tonal range is lost (the dark tones block up), the edges of the smoke become harder, and the noise is exaggerated.

Exposing for greenscreens in film

Figure 1

Now imagine that instead of a white screen, we’re shooting the smoke plume against a green screen and that the measured green brightness is the same as before. What’s the best exposure for the green screen? Obviously, it’s the same as before. The only difference is that the red- and blue-sensitive layers aren’t exposed. Just like in the smoke plume, greenscreen foregrounds potentially contain a full range of transparencies, from completely opaque to barely there. Transparent subject matter can include motion blur, smoke, glassware, reflections in glass windows, wispy hair, gauzy cloth, and shadows.

To reproduce the full range of transparency, the green screen should be fully exposed but not overexposed. In other words, its brightness should match the green component of a well-exposed white object like a white shirt, roughly defined as the whitest white in the foreground that still has detail. (It’s not desirable to expose that white shirt as top white, because it’s necessary to leave some headroom for specular reflections, on the shoulder in film, 100% and over in video.)

Excerpted from The Visual Effects Society Handbook- edited by Jeffrey Okun and Susan Zwerman © 2010 Elsevier Inc. All rights Reserved

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