Photography

Aperture Priority

Aperture Priority, symbolized as Av is one of the semi-manual exposure modes of a camera. Using Aperture Priority, the user can decide which aperture opening to use for a particular scene and the camera will compute for the best shutter speed suited for the scene. The selection of the shutter speed is based on the chosen aperture value, the sensor's ISO and the lighting on the scene. It still employs the same exposure metering algorithms used in auto mode, but this time the camera can only change the shutter speed.

This mode is used by photographers who want to control the Depth Of Field (DOF) of their shots. Depth of field refers to a range of distance from the camera at which the images can appear focused. Images that fall outside of this range will start to blur. Being able to control the depth of field is useful if you want your subject to be well focused while objects in front or behind the subject like the background is blurred. In a portrait shot this makes the subject stand out more and makes the picture more dramatic.

Small aperture values (large opening) gives a small depth of field. This makes the blurring of the background heavier and much easier to accomplish. Focusing is very critical if the depth of field is small, a slight variation could easily put your subject out of focus. Extreme cases of focusing problems due to small DOF can be encountered when doing Macro Photography where the subjects are usually very small and the DOF is measured in millimeters.

Large aperture values (small opening) makes the depth of field larger. This makes focusing easier and the background and foreground will tend to lose that blurring effect. This is not a disadvantage and is even valuable if you want to show as much details of a scene as possible like in a landscape shot. Even for portrait shots, choosing a larger aperture may give a better result as it can give enough blurring on the background to drive the attention more on the subject but not totally drowning its details to a point where it is no longer recognizeable.

Another practical use of large aperture values is for reducing image distortions due to lens limitations. A very common distortion is the Chromatic Aberration or color fringing where bands of colors apears on the edges of images in a scene with very high contrast. This is usually found at the long end of zooms and the larger the zoom, the more evident this distortion usually becomes. A large aperture value (small opening) tend to reduce and even eliminate chromatic aberration in many cases.