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Canon EOS 40D Guide to Digital Photography by David D. Busch

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How the EOS 40D Calculates Exposure

Your Canon EOS 40D calculates exposure by measuring the light that passes through the lens and is bounced up by the mirror to sensors located near the focusing surface, using a pattern you can select (more on that later) and based on the assumption that each area being measured reflects about the same amount of light as a neutral gray card with 18 percent reflectance. That assumption is necessary, because different subjects reflect different amounts of light. In a photo containing a white cat and a dark gray cat, the white cat might reflect five times as much light as the gray cat. An exposure based on the white cat will cause the gray cat to appear to be black, while an exposure based only on the gray cat ...

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