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

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Determining Exposure

Calculating the proper exposure for an electronic flash photograph is a bit more complicated than determining the settings by continuous light. The right exposure isn’t simply a function of how far away your subject is (which the 40D can figure out based on the autofocus distance that’s locked in just prior to taking the picture). Various objects reflect more or less light at the same distance so, obviously, the camera needs to measure the amount of light reflected back and through the lens. Yet, as the flash itself isn’t available for measuring until it’s triggered, the 40D has nothing to measure.

The solution is to fire the flash twice. The initial shot is a preflash that can be analyzed, then followed by a main flash ...

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