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

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Aperture Priority

In Av mode, you specify the lens opening used, and the 40D selects the shutter speed. Aperture Priority is especially good when you want to use a particular lens opening to achieve a desired effect. Perhaps you’d like to use the smallest f/stop possible to maximize depth-of-field in a close-up picture. Or, you might want to use a large f/stop to throw everything except your main subject out of focus, as in Figure 4.8. Maybe you’d just like to “lock in” a particular f/stop because it’s the sharpest available aperture with that lens. Or, you might prefer to use, say, f/2.8 on a lens with a maximum aperture of f/1.4, because you want the best compromise between speed and sharpness.

Figure 4.8. Use Aperture Priority to “lock in” ...

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