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Canon® EOS Rebel XSi/450D Guide to Digital SLR Photography by David D. Busch

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How the Rebel XSi Calculates Exposure

Your XSi calculates exposure by measuring the light that passes through the lens using a metering 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 will make the white cat washed out. Light-measuring devices handle this by assuming ...

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