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Lighting for Digital Part 19 - part 3 of 1 2 3

by Dave Montizambert Published 01/04/2011

How Dark Is Dark Enough:

How much you need to underexpose ambient light to eliminate its unwanted affect depends upon three things:

1. What fill-lighting ratio will be used. A brighter fill light will greatly minimise ambient contamination in shadows.
2. What contrast settings are used when processing RAW files. Increasing contrast in an image with contrast slider and/or tone curve will darken shadows making ambient contamination less evident.
3. What tones are present in the shadow areas of your set. A clean white object underexposed by 6 stops (measured with an incident meter), will still record and print with some detail, not much, but some. Typical densitometer readings (White Balance Tool) off a 6-stop underexposed white object in Adobe Camera Raw (ACR) or Lightroom (LR) are about 17 Levels for ACR and around 6.8% for LR. These readings assume RAW processing settings in ACR and LR are set to the manufacturer's default and a that 2.2 gamma colour space such as Adobe RGB 1998 and sRGB selected.
4. How reflective your subject is. Seven stops under is enough for eliminating the ambient light's effect on the various tones within the photo-set, but it might not be enough to cancel-out reflections (specular highlights) from the ambient source on reflective surfaces within the set. Humans, for the most part, are not very shiny, except for their eyes and jewellery, and so this is where you usually see the unwanted ambient light reflections such as a dim green line caused by a fluorescent tube. To be sure, it is best to do a visual check by taking a preliminary exposure and checking it out on screen, either zoomed-in on your camera's LCD or, better still, on your computer screen.

Unwanted reflections aside, it's a safe bet to underexpose the ambient light by 7 stops (measured with an incident meter), but not always necessary since ambient light contamination is usually only visible in the shadows. If you are going for low shadow contrast (lighter shadows), you can often get away with as little as three stops underexposure for the ambient light since a brighter fill-light generally overpowers the ambience in the shadows. However, having said that, the safest bet is to turn out the house lights and if windows are present, pull the blinds.

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1st Published 01/04/2011
last update 09/12/2022 14:55:52

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