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

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Improved Cross-Type Focus Point

One improvement that new Canon EOS Rebel XSi owners sometimes overlook is the upgrade to a cross-type focus point at the center position. Why is this important? It helps to review exactly how the XSi 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 left in the extreme enlargement shown in Figure 5.10, the transitions between pixels are soft and blurred. Even the boundary between the roof tiles and their light background is smudged. When the image is brought into focus (right), the transitions are sharp and clear. Although this example is a bit exaggerated ...

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