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

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Phase Detection vs. Contrast Detection

This discussion of phase detection versus contrast detection may be a little too techie for some readers, but if you really want to understand how autofocus works in Live View, it’s a worthwhile topic to consider. Extra details like this are why you bought this fat book instead of a pocket-sized guide to the Rebel XSi.

  • Phase detection. In this mode, the autofocus sampling area is divided into two halves by a lens in the sensor. The two halves are compared, much like (actually, exactly like) a two-window rangefinder used in surveying, weaponry, and non-SLR cameras like the venerable Leica M film models. The contrast between the two images changes as focus is moved in or out, until sharp focus is achieved ...

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