Aperture priority, often abbreviated A or Av (for Aperture value) on a camera mode dial

Mode dial

A mode dial or camera dial is a dial used on digital cameras to change the camera's mode. Most digital cameras, including dSLR and SLR-like cameras, support modes, selectable either by a dial or from a menu. On point-and-shoot cameras which support modes a range of scene types is offered. On dSLR...

, is a setting on some cameras that allows the user to choose a specific aperture

Aperture

In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...

 value while the camera selects a shutter speed

Shutter speed

In photography, shutter speed is a common term used to discuss exposure time, the effective length of time a camera's shutter is open....

 to match. The camera will ensure proper exposure

Exposure (photography)

In photography, exposure is the total amount of light allowed to fall on the photographic medium during the process of taking a photograph. Exposure is measured in lux seconds, and can be computed from exposure value and scene luminance over a specified area.In photographic jargon, an exposure...

. This is different from manual mode, where the user must decide both values, shutter priority

Shutter priority

Shutter priority refers to a setting on some cameras that allows the user to choose a specific shutter speed while the camera adjusts the aperture to ensure correct exposure...

 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

Depth of field

In optics, particularly as it relates to film and photography, depth of field is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image...

. Aperture priority is useful in landscape photography

Landscape photography

Landscape photography is a genre intended to show different spaces within the world, sometimes vast and unending, but other times microscopic. This popular style of photography is practiced by professionals and amateurs alike. Photographs typically capture the presence of nature and are often free...

, 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

Portrait photography

Portrait photography or portraiture is the capture by means of photography of the likeness of a person or a small group of people , in which the face and expression is predominant. The objective is to display the likeness, personality, and even the mood of the subject...

, 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

Landscape photography

Landscape photography is a genre intended to show different spaces within the world, sometimes vast and unending, but other times microscopic. This popular style of photography is practiced by professionals and amateurs alike. Photographs typically capture the presence of nature and are often free...

 a user would select a small aperture when photographing a waterfall

Waterfall

A waterfall is a place where flowing water rapidly drops in elevation as it flows over a steep region or a cliff.-Formation:Waterfalls are commonly formed when a river is young. At these times the channel is often narrow and deep. When the river courses over resistant bedrock, erosion happens...

, 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

Angular resolution

Angular resolution, or spatial resolution, describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object...

with relatively medium-sized apertures.