Aperture Priority And Exposure

Aperture Priority ModeOne way to take control of your photography output is to move the mode dial off “P” or “Auto”. These settings are fine for beginners or if you are just getting used to a new camera, but there comes a time when your confidence has grown, that you will find the need to have more control.

Aperture Priority (Av or A) allows you to change everything you need in order to have the creative exposures you dream of.

What Is Aperture

Aperture refers to the size of the camera’s lens opening when a picture is taken. The aperture determines how much light gets to the camera’s image sensor to “expose” the subject of the picture.

To get a better understanding of aperture, imagine you are in your bedroom and the only light source is a single window. If the window is very small, only a little light can get into the bedroom. If the window is larger, more light can enter the room.

young photographer - set on "auto"
This young photographer does not yet know about Aperture Priority.

This is the case with the lens opening of your camera. The smaller the aperture, the less light can get to the sensor. The larger the aperture, the more light gets to the sensor.

The key is to make sure just the right amount of light is allowed to enter through the lens opening. If the aperture is too small and not enough light is allowed in, the picture is under-exposed with excessive dark areas. If the aperture is too big and too much light gets in, the picture is over-exposed and washed out.

Aperture is measured in f-stops. These can be a bit confusing because the numbers are exactly opposite of what you might expect. The larger numbers, such as f/22 mean that the aperture is smaller. The smaller numbers, such as f/2.8 mean the aperture is larger.

Using Aperture Priority

When you set your camera on aperture priority, you are controlling only the f-stop of the lens. As you determine the f-stop, the camera will set the correct corresponding shutter speed. So, in truth, you can use Aperture Priority to control the shutter speed as well.

Want to freeze action? Choose a large aperture and the camera will automatically choose a faster shutter speed.

Want a slow shutter speed to blur the motion (as in a waterfall or whitewater creek)? Choose a small aperture and let the camera take care of the shutter speed.

f-stop is f/9

Notice the f-stop on this Canon Rebel T3i is f/9.0

Here are some guidelines for setting aperture according to scene requirements:

  • Shallow depth of field (for blurred backgrounds) – use a large aperture (small f-stop number).
  • Deep depth of field – use a small aperture (large f-stop number).
  • Freezing action – set a large aperture so that you get a fast shutter speed.
  • Blurring action – set a small aperture so that you get a slow shutter speed.

This method of keeping you camera on Aperture Priority will give you lots of control while allowing you to get the creative photos you desire. You get consistently great results with the least amount of effort or knowledge.