THE VES HANDBOOK

How to Expose a Green Screen Shot, and Why

By BILL TAYLOR, ASC, VES

Edited for this publication by Jeffrey A. Okun, VES Abstracted from The VES Handbook of Visual Effects – 3rd Edition Edited by Jeffrey A. Okun, VES and Susan Zwerman, VES

Figure 3.8: Schematic H&D curve. This graph shows how the color negative responds to increasing exposure. In film, each color record has a linear section, where density increases in direct proportion to exposure, and a “toe” and a “shoulder” where shadows and highlights respectively can still be distinguished but are compressed. The “white point” is shown for all three records: the density of a fully exposed white shirt that still has detail.

Figure 3.9: Normal smoke plume with ND filter strip, underexposed plume and strip with contrast boosted.

Images courtesy of Bill Taylor, ASC, VES.

Balancing Screen Brightness to the Shooting Stop

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, for example “one stop below key.” 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 green density on the negative, or at the sensor, in the backing area for every shot at any f-stop.

The ideal green density is toward the upper end of the straight-line portion of the H and D curve, but not on the shoulder of this curve, where the values are compressed. Figure 3.8 shows an idealized “H&D curve.” Eight stops of exposure range can comfortably fit on the H&D curve, a range recently exceeded by digital cameras.

Imagine a plume of black smoke shot against a white background. It is a perfect white: The measured brightness is the same in red, green and blue records. For clarity, the range of transparencies in the smoke has been duplicated with a stepped series of neutral density filters.

The densities in Figure 3.9 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 is 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 of the dark values in the smoke up off the toe. If the backing was overexposed, the thin wisps would be pushed onto the shoulder, 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 compensate for underexposure by boosting the image contrast or making a levels adjustment (remapping the brightness range to a wider gamut). As Figure 3.9 shows, boosting contrast makes the backing clear again and the blacks opaque, but tonal range is lost: the dark tones block up, the edges of the smoke become harder and noise is exaggerated. The effect of a levels adjustment is subtler (and preferable), but extreme levels adjustments betray the missing tonal values with banding or blocking.

Now imagine that instead of a white screen, we are shooting the smoke plume and the neutral density steps against a greenscreen and that the measured green brightness is the same as before. What is the best exposure for the green screen? Obviously, it is the same as before. The only difference is that the red- and blue-sensitive layers are not exposed. Just like the smoke plume, greenscreen foregrounds potentially contain a full range of transparencies. 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 greenscreen should be fully – but not over – exposed. 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. (We do not want to expose that white shirt as top white, because we want to leave some headroom for specular reflections, on the shoulder in film, 100% and over in digital recording.)