Aperture Priority

How to use aperture priority, AV (for Canon) or A (for Nikon).
Aperture, (AV or A mode) determines whether part of the image (photograph) is sharp and in focus, or if all of the image is sharp. Photographers can choose to have a sharp foreground and background, or they can blur the background by adjusting the diameter of the aperture (which is measured in f-stops).
The aperture controls the amount of light that reaches a digital camera sensor. An aperture acts much like the pupil of an eye, it opens wider as light decreases to let in more available light into the camera’s sensor and it gets smaller when light increases to reduce the amount of light entering the sensor. Aperture and shutter speed are closely related and will effect the exposure of your image. The faster your shutter speed is, the larger the opening of your lens will be and visa versa.

On SLR digital cameras, aperture is indicated by an (f-stop) f-number value. The diameter of an aperture is measured in f-stops. A lower f-stop number opens the aperture and admits more light onto the camera sensor. Higher f-stop numbers make the camera’s aperture smaller so less light hits the sensor. When an aperture is opened up by one f- stop, the amount of light which reaches the sensor is doubled. F-stops are expressed in three different ways: f/8, f-8, 1:8.

Aperture settings can be used creatively to control your depth of field, how much of an image is clear / sharp in front and back of the focus point (main subject). For example, using a wide aperture (small f-stop number) is a desirable technique for many close-up and portrait shots. This will keep the foreground and focus point (subject) in clear / sharp focus and will blur the background of the image. Important Note - When using aperture priority mode, when you change the aperture (F-stop value) the shutter speed is automatically changed to maintain proper exposure.