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

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Improved Cross-Type Focus Points

One improvement that new Canon EOS 40D owners sometimes overlook is the upgrade to cross-type focus points at all nine focus zone positions (with lenses having that f/5.6 or larger maximum aperture), plus a center cross-type focus point that is extra sensitive when used with lenses having a maximum f/stop of f/2.8 or better. Why is this important? It helps to review exactly how the 40D determines focus.

The camera looks for contrast between adjacent pixels to determine relative sharpness—specifically, the transitions between those groups of pixels that determine the edges in a subject. At top in the extreme enlargement shown in Figure 5.10, the transitions between pixels are soft and blurred. Even the boundary ...

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