Changing the aperture setting on your camera has a direct effect on the exposure on your image. But it also effects your image in another way. The aperture setting creates what’s called depth of field.

When you take a picture, you focus on a specific point. This would typically be your subject; let’s say a person. As you move away from the focal point – either closer to the camera or further from the camera – the picture moves out of focus. The depth of field describes how big of a range is in focus, and how quickly the picture moves out of focus.

Large Aperture, Small Depth of Field

As the aperture gets bigger, your depth of field decreases. The picture to the left was taken with a relatively large aperture, f/2.8.

The focal point here is somewhere between the two girls faces. Ideally, it would have been focused on one or the other, but the student who took this picture missed the focus a bit. Doh.

Nonetheless, the two faces are relatively in focus, although the girl in the back is beginning to get a little blurry. Beyond her, though, is hopelessly out of focus. The wall is one giant blur. Notice, too, that the foreground of the picture (the gray hood) is also blurred and out of focus. There’s a very small range – about 2 ft here – that’s in focus, and the rest of the picture becomes increasingly blurry.

Small Aperture, Large Depth of Field

This picture was taken on a relatively bright day. Since there was plenty of light around, it was possible to use a smaller aperture (the previous photo was taken indoors, where the amount of light was severely limited). The picture was shot at f/10.

In this case, there is a much larger depth of field. The focal point of the original picture (this was cropped) is somewhere around #53 in the back. However, notice that the running back in the foreground and the linebacker to the left are both in relatively good focus. They’re slightly out of focus, but the depth of field is large enough that they are fairly crisp.

The larger problem with this image is that there’s a little bit of motion blur/camera shake. It was shot at 1/125th of a second, which isn’t quite fast enough to freeze the action at a football game.

So Do You Want a Large or Small Depth of Field?

Well that depends on the type of image you’re trying to create. There are times when each is an appropriate choice.

A common use for a small depth of field is portraiture. By using a small depth of field, the subject (the person in the portrait) is held in tight focus, while the background becomes extremely blurry. This directs the viewers attention to the subject. Similarly, sports photographers often use a relatively small depth of field. This way your subject is in focus but the opposing team’s bench is not.

A common use for a large depth of field is group portraiture. If you’re shooting a large group and you use a small depth of field, only one or two people will be in crisp focus. The rest of the group will fall out. The larger the group, the larger depth of field you’ll want. Landscape photographers also often utilize a large depth of field, since the scenes they capture are so large.