Among the shooting modes of the Sony Alpha line of digital SLR cameras is aperture-priority, which allows the user to change the amount of light coming through the lens. The photographer sets the aperture, also called an f-stop. The camera then sets the appropriate shutter speed according to the lighting conditions. Shooting in aperture-priority gives the photographer control over the depth of field of the image. A lower f-stop means more light is coming through the lens. With more light, the less depth of field an image has -- the subject and foreground will be in focus and the background will blur.
Turn on the camera.
Move the mode dial to "A" to activate the aperture-priority setting.
Twist the control dial, found on the top-right corner of the camera, to select the desired aperture. Find the current aperture value on the top-right corner of the LCD monitor, indicated next to the "F."
Look through the viewfinder while pressing the depth of field preview button, found just below the lens, to see how your picture will appear.