What Does the Aperture Setting of a Camera Do?

There are three basic settings that affect the exposure of your picture - aperture, shutter speed, and ISO rating. Combined, these three settings determine how much light is allowed into your camera.

You need to balance all three settings to let in the right amount of light. Too much light leads to an blindingly bright, over-exposed photo. Too little light leads to a dark, under-exposed photo. To make things more complicated, each of these settings has a unique side effect that can impact the final look of your picture.

So what does the aperture setting do, and how does it affect your photo?

Aperture - The Size of a the Hole Letting Light In

Think for a moment about the pupils in your eyes. Unconciously, your brain changes the size of your pupils quite often to adjust to the amount of light that you are exposed to.

If you're in a dark area, your pupils will dilate - or grow larger. This allows more light in. Conversely, if you are in a very bright environment, your pupils will shrink. This limits the amount of light that comes in.

The aperture of a camera works in the same way. There's an circular opening in your camera that allows light in (onto film for a film camera or onto the sensor for a digital camera). The larger the opening, the more light and the smaller the opening, the less light.

This concept translates into a counter-intuitive number on your cameras setting. The aperture is measured in "f-stops," and you'll usually see the aperture rating next to a letter "f" in your camera menu. A high number (i.e. f11 or f22) indicates a small aperture and less light coming in. A low number (i.e. f1.8 or f2.8) indicates a large aperture and more light coming in.

By changing the aperture and making the hole larger or smaller, you adjust the amount of light that comes in. Combined with the shutter speed and the ISO rating, this allows you to capture the perfect amount of light for a well-exposed photo.

What About that Side Effect?

In theory, any scene can be captured by a variety of camera settings. If you open the aperture wider, you can increase the shutter speed or lower the ISO rating to keep the amount of light constant. Or, if you shrink the aperture, you can decrease the shutter speed or increase the ISO rating to maintain a constant amount of light.

But, each of these settings brings with it a side effect. If you control the amount of light, a picture taken at f1.8 (a large aperture) will be very different from one taken at f22 (a really small aperture).

For the aperture setting, this side effect is the depth of field. In other words, the area of the picture that's in focus.

If you use a small aperture, you've got a really large depth of field. At f22 (the smallest aperture), you'll be able to get a whole lot of stuff in focus. This is really great for landscape photography, where you want to capture areas of the image that are both close to your camera and far from your camera. This is also useful for portraits of large groups, since the people on the outside are likely to be farther from the camera than people in the center.

If you use a larger aperture, on the other hand, you'll have a really small depth of field. A subject that's 3 meters from your camera might be in focus, but an object that's 4 meters away will be hopelessly blurred out of focus. Why would you want something to be out of focus? To highlight your subject! This is a common technique in portraits of individual people. By using a small depth of field, you can blur out the background and make the viewer focus immediately on your subject.

Aperture Priority Mode?

One of the modes on your camera is aperture priority mode - usually labeled with Av or A. This means that you pick the aperture setting, and the camera automatically chooses the shutter speed and ISO rating to properly expose the photo (allow enough light in).

When should you do this? Do this when you want to control the depth of field. If you want a really shallow depth of field for a portrait, use aperture priority mode to set the aperture to f1.8. If you want a large depth of field to capture a landscape, use apterture priority mode to set the apterture to f11 or f16.

But, you'll also need to be cognizant of the side effects of the other settings. While your camera will automatically pick a shutter speed to allow enough light in, you might not be happy if that means a shutter speed of 1 second or an ISO rating of 6400. Even if you're not using the manual mode of your camera, you need to know how these three settings interact to achieve a well exposed photo.