The Aperture-Priority AE Mode is a semi-automatic mode that allows you, the photographer, to tell the camera which aperture you want it to use — and let it automatically determine the appropriate shutter speed for a correct exposure.
A “correct exposure” is simply a shutter speed and aperture combination determined by your camera’s light meter that will result in your image sensor receiving enough light to properly record your scene.Too much light and you get an “over-exposed” picture; too little light, and you get an “under-exposed” picture.
All the fancy technology in your camera (and some cameras list a lot of features) whirl and twirl… and eventually return just two values: an aperture (how big a hole to open to let light in) and shutter speed (for how long to keep that hole open).
In effect, you have taken control of the aperture setting. Everything else is on automatic, but you decide which aperture you want the camera to use depending on what you want to achieve in your picture.
The way you typically change the aperture when in Aperture-Priority AE mode is by rotating the Command Dial. You use your thumb to rotate the Command Dial and you will see the aperture change on the screen.
Some cameras also have a Sub-command Dial, usually in front of the camera at the top of the handgrip and just below or above the shutter release button.
Check your User’s Manual to see how your camera is set and whether you need to rotate the [Main] Command Dial or the Sub-command Dial to change aperture in Aperture-Priority Mode.
As you rotate the [Main] Command Dial (or Sub-command Dial, depending on your camera), you can see the aperture changing in the viewfinder. Here, the shutter speed (1/500 sec.) and aperture (f/5.6) are visible below the scene you’re taking.
Some cameras also allow you to view all the settings on a Command Info screen, as above. As you rotate the [Main] Command Dial (or Sub-command Dial, depending on your camera), you can see the aperture changing on the Command Info. Here, the shutter speed (1/320 sec.) and aperture (f/7.1) are displayed on the LCD screen.
Depth of Field
Just what do you want to achieve in your picture by selecting a particular aperture over another?
You want to control the depth of field.
Depth of Field is a fancy word that simply says how much of the picture you want to be sharply in focus.
Here’s an example of a picture with lots of depth of field using an aperture of F9.0. Notice how everything, from near to far, appears sharp in the picture. When taking landscape photographs, you would usually want this effect. And the way to achieve lots of depth of field using a DSLR is to use a small aperture, e.g. f/16.
It is not always possible to use the smallest aperture available on your camera’s lens, so you have to use the smallest that gives a correctly exposed picture. In the example above, I used F9.0.
An aperture has an associated “aperture value”. That’s what the number following the “f/” or “F” means. So an aperture of f/5.6 (or F5.6) has an aperture value of 5.6. An aperture of f/16 (or F16) has an aperture value of 16.The only thing you have to remember here is that the larger the aperture value, the smaller the aperture. The smaller the aperture value, the larger the aperture.
So, f/16 (F16) is a small aperture and f/2.8 (F2.8) is a large aperture.
Here’s an example of a picture with shallow depth of field using an aperture of F5.0. Notice how only the main subject is sharply in focus and the background has been nicely thrown out of focus. When taking portraits, you would usually want this effect so your main subject stands out from a [potentially] distracting background. And the way to achieve a shallow depth of field using a DSLR is to use a large aperture, e.g. f/2.8.
It is not always possible to use the largest aperture available on your camera’s lens, so you have to use the largest that gives a correctly exposed picture. In the example above, I used F5.0. The blurring effect of the background would have been even more pronounced were I able to use a larger aperture.
So, if you have been wondering why your landscape pictures did not come out sharp all over or why your main subject did not have the same punch as the examples above, now you know! When set to AUTO, your camera selects both the aperture and shutter speed and does not really know what effect you want to achieve. You tell it from now on, and control how your pictures come out!
Article Source: http://www.photoxels.com/using-the-aperture-priority-ae-mode/