When you switch your camera to aperture priority you are telling it that you are going to decide on the aperture that used to take your photograph; the camera will then set the shutter speed automatically, so you achieve the correct exposure overall, more than this, when you choose the aperture, you’re deciding what depth of field do you want in your shot. This is one of the most important creative decisions you can make when taking a photograph.
Setting a wide aperture opens ups the iris to allow more light through the lens. However, it also reduces the depth of the field on appearing sharp: almost everything behind and in front of this point will be blurred. This is great when you want to conceal a distracting background.
A medium aperture setting offers the best compromise between the amount of light passing through the lens and the depth of field. In addition, the lens usually delivers its best performance in terms of image sharpness at medium aperture setting.
Setting a small aperture restricts the amount of light passing through the lens. It also increases the depth of field; so much more of the scene appears in focus: if you use a very small aperture setting is possible to get everything looking sharp.
What effects can I achieve?
The main reason for controlling the aperture is because it allows you to determine the depth of field. This is the amount of the scene in front of and behind the actual focus point that appears sharp in the final image. Using a small aperture increase the depth of field, so more your scene appears sharp but it often results in longer shutter speeds, which may mean that you will need to use a tripod. Wide aperture setting reduces the depth of field and helps ensure faster shutter speed, but you need to bear in mind that choosing where to focus becomes even more important than usual.
Using Aperture Priority Mode to Control Depth of Field
Aperture size works with shutter speed to determine the depth of field, which is the difference between a good photo and a great photo. Imagine a landscape shot the first few inches of the image are sharps.
To select aperture priority mode, look for the A or AV on the mode dial on the top of your DSLR or advanced point-and-shoot camera.