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Aperture priority, often abbreviated A or Av (for Aperture value) on a camera mode dial, is a setting on some cameras that allows the user to choose a specific aperture value while the camera selects a shutter speed to match. The camera will ensure proper exposure. This is different from manual mode, where the user must decide both values, shutter priority where the user picks a shutter speed with the camera selecting the aperture to match, or program mode where the camera selects both.
The main purpose of using aperture-priority mode is to control the depth of field. Aperture priority is useful in landscape photography, where a narrow aperture is necessary if objects in foreground, middle distance, and background are all to be rendered crisply, while shutter speed is often immaterial. It also finds use in portrait photography, where a wide aperture is desired to throw the background out of focus and make it less distracting.
Another common use of aperture priority mode is to suggest how the camera should determine a shutter speed, without risking a poor exposure. In landscape photography a user would select a small aperture when photographing a waterfall, hoping to allow the water to blur through the frame. When shooting a portrait in dim lighting, the photographer might choose to open the lens to its maximum aperture in hopes of getting enough light for a good exposure.
In addition, aperture priority mode allows the photographer to force the camera to operate the lens at its optimum apertures within its aperture range for a given focal length of the lens. Commonly, lenses provide greatest resolving power with relatively medium-sized apertures.
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