Controlling your depth of field

Depth of Field (DOF) refers to the distance between the farthest and nearest points which are in focus. It can also be identified as the zone of acceptable sharpness in front of and behind the subject. In the picture of Nemo, above, you’ll notice that in the left hand shot he’s totally in focus, from gills to tail fin. In the right hand shot, his tail fin is blurry.

When you take a picture, there are several factors which can control or impact your depth of field. Less depth of field is called “small” or “narrow” or “shallow”. Having great depth of field is called a “wide” or “large” depth of field.

There are lots of reasons why you may want to control the depth of field, and make it narrower or wider. Most of these are because of artistic decisions, when you want to force attention to one part of the picture, or when you want to obscure another.

Aperture

A wider aperture (small f/number) causes a narrower depth of field. A smaller aperture (big f/number) causes a wide depth of field. So, to blur out the background use a f/stop like f/2.8 or f/4. To get everything in the scene focus select f/16 or f/22.

Sweet Spot

Remember though, that your lens has a sweet spot and if you select an aperture that is too wide or too small, you won’t be as sharp as you could be.

Focal Length

Another way to control your DOF is through the lens choice. Longer focal lengths (200mm or 300mm) tend to have a narrower depth of field, whereas wider lenses (35mm or 50mm) have a greater depth of field.

Focal Point

Remember that DOF is extends beyond the focus point. Typically it extends further behind the focal point than in front, so if you’re trying to blur the background you may want to bring the focus in a little ways.

There you have it. Choosing a depth of field and attaining it relies on your choice of all those variables. You can usually use your DOF Preview button on your camera (most new SLR’s have it) to get a good idea of what to expect, but until you see the final shot it’s always tough to tell what it will look like.