Depth of field indicates how much of your scene is in focus, both in front of the subject and behind it. One definition explains depth of field as the distance between the nearest and farthest points in a scene that appear acceptably clear in an image.  That really doesn’t explain much.

We prefer to use the term ‘out of focus.’ Sometimes you want to blur the background so the focus is on the main subject. Sometimes you want to blur the foreground. We call this controlling the depth of field. To understand how to control the depth of field, we must consider the three factors that control the depth of field.

The three factors that control the depth of field are 1) aperture, 2) focal length, and 3) the distance from the subject to the camera.  In previous lessons we covered aperture and focal length. The focal length is determined by the lens and can’t be changed. The aperture can be changed, limited by the lens you’re using.  What about the distance from the subject to the camera? Sometimes it can be changed. You can move closer to the subject or move farther away. However, if you’re standing on a bridge taking pictures of the waterfall in front of you, chances are you can’t do much about the distance to the subject.

To create a deeper field of depth you can use a larger aperture number (smaller aperture opening), use a lens with a shorter focal length, or move further from the subject. To create a shallower depth of field you can use a smaller aperture number (larger aperture opening), use a lens with a longer focal length, or move closer to the subject.

A lens with a long focal length will not allow you to achieve a shallow depth of field. You will not be able to achieve a shallow depth of field if you’re a long distance from the subject. A lens with a short focal length will not allow you to achieve a deep depth of field.  And so on, and so on, and so on. Some things in photography have very little room for change.

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