Similar to Shutter Priority in Part 2 of this article series, Aperture Priority is a semi-automatic photographic shooting mode that is in the creative range of all DSLR cameras and bridges between fully automatic and manual exposure settings.

It is usually shown on the mode selection dial of your camera as 'A' or 'Av' and it enables you to select an aperture of your choice and then the camera automatically selects the correct shutter speed to match the light meter reading on the camera.
 
Aperture Priority enables you to very simply vary the 'depth of field' - this is the area of your photograph that is at an acceptable sharpness to the eye both in front of and behind the subject upon which you focus. The smaller the aperture you select, the larger the range of sharp focus and conversely the larger the aperture, the narrower the zone of sharp focus.
 
The Aperture Priority function allows you to really focus on the creative outcome you are trying to achieve, without having to concern yourself too much with other settings. Wide apertures (smaller numbers e.g. f2.8) are often great for portraits as you can throw the background out of focus, while the subject of your photo can be perfectly sharp. On the other hand, you will often want a very narrow aperture (bigger numbers e.g. f14) when shooting landscapes as you want to capture as much in-focus detail as possible.
 
You will, however, need to keep an eye on your shutter speed because as you change your aperture, your shutter speed will also change. This can result in blurred images from camera shake, particularly if you are shooting with the camera in your hand rather than on a tripod.

Similarly, under or over exposure is another issue to watch out for, but your DSLR should flash a warning in the viewfinder or on the LCD to bring it to your attention. Be on the lookout for this warning!
 
When light levels fall, particularly towards evening or on a cloudy day, the photographer gradually opens up the aperture, and eventually opens it as wide as possible in order to maintain a suitable shutter speed so as to avoid the photo from being out of focus. However, depth of field is lost as a result and this is accentuated if using a telephoto lens or a macro lens. The depth of field can become too narrow to ensure precise focusing, often reducing it down to just a few millimetres. If this occurs, it is best to increase the ISO setting in order to increase shutter speed with the logic being that it is better to have more 'noise' in the photo than no usable photo at all.

A handy feature that should co-exist with the Aperture Priority mode on your camera is Exposure Compensation. This feature allows you to quickly correct the exposure if the lighting surrounding you deceives your light meter (e.g. if you are shooting a portrait on a sunny day on the beach, you may need to push the exposure compensation up to ensure that the face of your subject is bright enough).

To assist you further in ensuring that your subject is in focus, a "depth of field" button can be found on DSLR cameras at the front of the camera, adjacent to the lens. This function allows you to determine if your subject is 'in focus' from the front to the back. Pressing this button stops the lens down to the selected aperture to give you a preview of the depth of field within the viewfinder.  In bright conditions this is easily seen, however in low light levels and in narrow aperture settings it is difficult to assess the zone that is in sharp focus.

As you can see from this article, aperture priority is an extremely useful camera function, one that you would do well to fully explore to ensure that you are able to continually satisfy your clients with creative photographic results.

.