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

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Making Sense of Sensors

The number and type of autofocus sensors can affect how well the system operates. As I mentioned, the Canon EOS 40D has nine AF points. The new Digital Rebel XTi has nine, too. The original Digital Rebel and Digital Rebel XT had seven autofocus points, and Canon’s high-end cameras, like the 21MP Canon EOS-1Ds Mark III has a whopping 45 autofocus points. These focus sensors can consist of vertical or horizontal lines of pixels, cross-shapes, and often a mixture of these types within a single camera, although, as I mentioned, the EOS 40D includes cross-type sensors at all positions. The more AF points available, the more easily the camera can differentiate among areas of the frame, and the more precisely you can specify ...

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