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

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C.Fn III-01: How the Autofocus System Behaves when Autofocus Fails

Lens drive when AF impossible

This setting controls how the 40D’s autofocus system handles hard-to-autofocus (or impossible-to-autofocus) situations. Canon’s AF system operates on differences in contrast: When a subject displays the greatest amount of contrast, it is deemed to be in sharp focus. When a scene has little inherent contrast (say, a blank wall or the sky) or if there isn’t enough illumination to allow determining contrast accurately (in low light levels, or with lenses having maximum apertures of less than f/5.6) a lens may be unable to achieve autofocus. Very long telephoto lenses suffer from this syndrome because their depth-of-field is so shallow that the correct ...

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