#Aperture Priority Auto Exposure
Aperture Priority Auto Exposure is a semi-automatic shooting mode. The user sets the aperture and the camera meters the scene to determine the best shutter speed for a correct exposure. ISO can either be set by the user or used in automatic mode so that the only variable not controlled by the camera, rather than photographer, is aperture. It is useful for maintaining a desired aperture look, like a particular depth of field, while being able to move quickly with variances in a scene like overall brightness or darkness of what's in the frame or changing light.