Aperture Priority

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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.

Aperture priority can even be used to specify a range of shutter speeds you want to use under varying lighting conditions, which seems almost contradictory. But think about it. You're shooting a soccer game outdoors with a telephoto lens and want a relatively high shutter speed, but you don't care if the speed changes a little should the sun duck behind a cloud. Set your 40D to Av, and adjust the aperture until a shutter speed of, say, 1/1,000th second is selected at your current ISO setting. (In bright sunlight at ISO 400, that aperture is likely to be around f/11.) Then, go ahead and shoot, knowing that your 40D will maintain that f/11 aperture (for sufficient DOF as the soccer players move about the field), but will drop down to 1/750th or 1/500th second if necessary should the lighting change a little.

A blinking 30 or 8,000 shutter speed in the viewfinder indicates that the 40D is unable to select an appropriate shutter speed at the selected aperture and that over-and underexposure will occur at the current ISO setting. That's the major pitfall of using Av: you might select an f/stop that is too small or too large to allow an optimal exposure with the available shutter speeds. For example, if you choose f/2.8 as your aperture and the illumination is quite bright (say, at the beach or in snow), even your camera's fastest shutter speed might not be able to cut down the amount of light reaching the sensor to provide the right exposure. Or, if you select f/8 in a dimly lit room, you might find yourself shooting with a very slow shutter speed that can cause blurring from subject movement or camera shake. Aperture Priority is best used by those with a bit of experience in choosing settings. Many seasoned photographers leave their 40D set on Av all the time.

Figure 4.8

Use Aperture Priority to "lock in" a large f/stop when you want to blur the background.

Figure 4.8

Use Aperture Priority to "lock in" a large f/stop when you want to blur the background.

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Digital Camera and Digital Photography

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