Aperture priority

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 an appropriate aperture, or program mode where the camera selects both.

Aperture priority is therefore useful in landscape photography, for example, where it may be desired that objects in foreground, middle distance, and background all be rendered crisply, while shutter speed is immaterial.

To obtain this large depth of field, a narrow aperture (identified by a high f-number, e.g. f/16 or f/22) is necessary.

[1] Another common use of aperture priority mode is to indirectly affect shutter speed for a desired effect.

In landscape photography, a user might select a small aperture when photographing a waterfall, so that the camera will select a slow shutter speed (to allow a sufficient amount of light to reach the film or sensor for proper exposure), thereby causing the water to blur through the frame.