Oct 15, 2004

What is depth of field?

Answer: A term that indicates how much of a scene will be in reasonably sharp focus. It is sometimes referred to as the acronym, DOF.

Depth of field is the zone of acceptably sharp focus in front of and behind a subject. For example, a picture in which the background is a soft blur has little depth of field. In another picture, both foreground and background elements appear to be in focus; this indicates more extensive depth of field. Aperture is the main factor that affects depth of field.

In some situations, you want to increase the depth of field. For example, in a scenic photo, choose a narrow aperture; a large f-stop number such as f/16 or f/22 so that much of the foreground and background will seem sharp in the photo. For other situations, you'll want to decrease depth of field. For example, in a portrait where you want the background to be blurred, choose a wider aperture, or a smaller f-stop number, such as f/2.8, f/4, or f/5.6. The subject will be in sharp focus, but the background will be "soft," and less distracting.

Tip Remember that large numbers enlarge and small numbers shrink. In other words, large f-numbers, such as f/22, enlarge the range of acceptably sharp focus. Small f-numbers such as f/4 shrink the range of acceptably sharp focus and help to blur the foreground and background.

When you choose a small aperture such as f/16, a longer shutter speed is required to ensure that enough light reaches the sensor for a correct exposure. With slower shutter speeds, however, camera shake or subject motion can result in blurred pictures. If the shutter speed is slow, use a tripod, or switch to a faster ISO setting that allows for faster shutter speeds. If you want to "freeze" a moving subject, you'll need to use a high ISO.

While aperture is the most important factor that affects the range of acceptably sharp focus in a picture, other factors also affect depth of field including:

1. Telephoto lenses have a narrow range of view, and they compress perspective as shown in this photo. (Photo taken at 1/160 sec., f/14, 300mm prime lens on a digital camera.)

2. Camera-to-subject distance. At any aperture (f-stop), the further you are from a subject, the greater the depth of field will be. If you take a scenic photo of a distant mountain, the foreground, midground, and background may all be in acceptably sharp focus. If you take a head-and-shoulders portrait, however, only your subject will be in very sharp focus.

3. Focal length of the lens. In simple terms, focal length determines how much of a scene the lens "sees." From the same shooting position, a wide-angle lens sees more of the scene than a telephoto lens. In addition, a wide-angle lens produces an extensive depth of field provided you are not extremely close to the subject. In more technical terms, a wide-angle lens or zoom setting has a 110-degree angle of view while a telephoto lens may have only a 23-, 12-, or even 4-degree (narrow) view of the scene. Because a long (or telephoto) lens or zoom setting (any lens or zoom setting over 50mm), magnifies the subject, the depth of field is shallow so that only the subject is in very sharp focus.