How and When to Use Aperture Priority

Aperture priority is a mode on your camera where you change the aperture (the opening in the lens) to your heart's content and the camera does the rest of the work for you.

You gain a small measure of control over your photos, but don't have to worry about getting a lot of bad exposures in the process.

How and when should you use aperture priority mode? Let's find out.

Setting Aperture Priority

You can set your camera to aperture priority mode by turning the control knob (located on the top of the camera) to the "Av" setting. If you then spin the main control dial (under your index finger or thumb) you will see the aperture numbers changing on the camera's display.

Let's run through a quick example here:

First, spin the dial until the number that you see on your camera's display for the aperture setting is 8 (a MEDIUM aperture). Take a picture. The photo is correctly exposed because the camera chose the right shutter speed to match the aperture that you selected. This is exactly the same as leaving the camera in AUTO mode, except that you have control over the aperture setting.
Now spin the control dial until the aperture reads 22 (a NARROW aperture). If you take a photo with an aperture of 22 in dim light it's likely to turn out blurry, because the camera will choose a very slow shutter speed to compensate for the narrow aperture.
Finally, spin that control dial one last time until the aperture reads 3.5 (a WIDE aperture). Even if you are indoors, an aperture this wide will help you take clear photos without having to use the flash as a light source.

This brings us nicely into our discussion of when you want to change the aperture.

When to Change Aperture

It's pretty straightforward:

In dim light select a small aperture number (wide opening) to let in plenty of light
In bright light select a large aperture number (narrow opening) to let in a reduced amount of light

This is the use of aperture at its most basic, but it's the foundation of more advanced techniques to come. If you've ever wondered how some professional photographers can shoot in almost non-existent light without using a flash, a wide-open aperture is just one of the tools that they are using.