What Is Depth of Field?

When we consider the timing of a shot, we begin to move from the technical aspects of focusing to its artistic application. When you begin to explore how the focus of an image influences its artistic expression, you must consider the image's depth of field.

 Ufizzi Courtyard
Nikon D2H
1/90 sec, f/4.8
EV -0.33
Focal length 31mm

Depth of field refers to what's in sharp focus and what's blurry. The basic tenet is that all images have an area of focus, measured in distance from the camera lens. Objects in the center of the focus field are in the highest degree of focus. Objects further from the focus point are out of focus. For example, if you set your camera to focus on a subject that's 10 feet away, that object becomes the center of the focus field. But what about the objects that are 8 feet from the camera...or 13 feet away...or 5 feet away? Are they also in focus, and does the focus effect fade gradually or stop abruptly? The answers to these questions are determined by how deep the focusing field is; that is, over what distance do things move from sharp focus to soft obscurity?

When an image has a deep depth of field, lots of things are in focus, and the transition between sharp and soft focus is a gradual one, if it happens at all. Figures and show the same shot of a carved pediment that stands 15 feet in front of a stone wall. Notice that in Figure , the wall and the pediment are in sharp focus, revealing a deeper depth of field. In Figure , the rear stone wall is out of focus because of the image's narrow depth of field.

 Deep Depth of Field
Nikon D2H
1/10 sec, f/3.4
Focal length 400mm

 Narrow Depth of Field
Nikon D2H
1/400 sec, f/5.6
Focal length 400mm

If the center of the focus field is 10 feet away, objects as close as 3 feet and as far as infinity might be tack sharp (in other words, everything in the image appears in sharp focus). Deep depth-of-field effects are often used in landscape images where you want the flowers in the foreground and the mountains in the distance to be as sharp as possible. A deep depth of field is also useful for still-life images, commercial photography, and other subjects for which the photographer wants to communicate all the detail present in the image.

Narrow depth of field means that the transition from the central focus point and the out-of-focus areas is quick and abrupt. If the subject is in focus at 10 feet away, the person standing next to that subject at 8 feet away might be soft and blurry. A narrow depth of field tends to isolate and emphasize the subject, rendering the subject sharply, while throwing everything else out of focus. Close-up photography is notorious for creating narrow depth-of-field images, where a macro lens can shift from sharp focus to a soft blur in just a few millimeters.

Controlling Depth of Field

When you focus your camera in a landscape, it quickly becomes apparent that the depth of the focusing field is not equally distributed in front of and in back of the central focusing point. More sharpness is apparent behind the focus point than in front, as caused by the way the optical field is compressed as it moves away from the camera. A standard rule of thumb is that the depth of field extends 1/3 in front of the focus point and 2/3 behind.

The variable that delivers the most control over depth-of-field effects is the aperture (f-stop) of the lens. When the aperture is reduced (set to a higher number), the depth of field increases (gets deeper). Thus, an aperture of f/22 has a much deeper depth of field than an aperture of f/5.6. Of course, as the aperture shrinks (the f-stop gets larger), the shutter speed slows down to let in more light. As a result, you'll find that you usually need a tripod for deep depth-of-field shots.

When you want to create an extremely narrow or deep depth of field, choose lenses with an extreme focal length. Short lenses, including wide angles and fish eyes, deliver a very deep depth of field. Long lenses such as telephotos generate a narrow depth of field. Thus, choose a 28mm lens for a deep depth of field or a 300mm lens for a narrow one.

The last thing to remember about depth of field is that the distance between the focus point and the camera determines how wide or narrow the field will be. This is why the focus markings on many camera lenses jump from a focus distance of 12 feet right to infinity while marking off much smaller increments between 3 and 6 feet. It's extremely critical to get the focus point just right when objects are closer to the camera.