Aperture-priority AE mode is shown on the camera Mode dial as Av. Av mode is a semiautomatic mode that enables you to control the aperture. In this mode, you change the aperture by turning the Main dial, and then the camera automatically calculates and sets the appropriate shutter speed. As detailed earlier, aperture is the primary factor that controls the depth of field. A wide aperture, such as f/3.5, provides a shallow depth of field that softly blurs the background. A narrow aperture, such as f/11, provides an extensive depth of field that shows both foreground and background elements reasonably sharp. For most day-to-day shooting, this is a good shooting mode because you can quickly control the depth of field by changing the aperture or f-stop.
Additionally, you may want to use Av mode to set and maintain the aperture at the "sweet spot" of the lens that you're using. The sweet spot is the aperture at which the lens provides the best detail, contrast, and sharpness, and it varies by lens. Or in low-light scenes, you may want to use Av mode because you know that you'll need to shoot consistently at the largest aperture.
2.9 This image was taken in Av mode with a wide aperture that provides a shallow depth of field and also places the concentration on the stem of the yellow pepper. Exposure: ISO 400, f/2.8, 1/00 second.
In Av mode, you have control over the AF mode, AF point, drive and metering modes, white balance, Picture Style, and use of the built-in or an accessory flash.
Tip You can preview the depth of field for an image by pressing the Depth-of-field preview button on the front of the camera. When you press the button, the lens diaphragm closes to the aperture that you've set so that you can see the range of acceptable focus.
If you choose this mode, then check the shutter speed in the viewfinder; if it is 1/30 second or slower, be sure to use a tripod or use the handholding rule provided earlier in the Tv section. Aperture value increments can be changed from the default 1/3-stop to 1/2-stop increments using C.Fn-1.
Reference For details on setting Custom Functions, see Chapter 5.
To change to Av mode, follow these steps:
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