Depth of Field

Donald W. Smith
March 2002

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If you're like most videographers, you want to make your shots look as good as possible. Managing which areas of your shots are in and out of focus can be one of the most valuable tools in your arsenal. And controlling the depth of field ultimately determines your focus area. By understanding how your camcorder's lens operates, and with a little practice, you will be on your way to more interesting and creative shots.

Depth of field is the area of an image that appears in focus. Camera settings, the amount of light and your equipment's technical specifications are all factors that can impact depth of field, making it deep or shallow.

First, it is important to understand that whenever a lens is involved, there is only one plane of exact focus. There is, though, an area of acceptable focus in front of and behind this plane. In a shallow depth of field, only the subject and a small area in front of and behind it will be in sharp focus. A deep depth of field results in objects farther away from the focus plane lying within acceptable focus range (Figure 1).

Controlling the depth of field allows you to make your subject stand out sharply from the background, or to put as much of the shot as possible in focus. For example, when shooting your son playing violin in the midst of the string section, you may want him to stand out from the rest of the orchestra. On the other hand, you might want the entire orchestra to be in focus, which may not be a trivial task in low-light situations. Most consumer-level camcorders have very deep depths of field in most shooting situations.

While there are many ways to alter your camcorder's depth of field, there are really only two variables that determine depth of field: focal length and aperture. Focal length is a function of your camcorder's lens and is changed when you zoom in or out on a subject. You can, therefore, easily change the depth of field of your shot by simply zooming. The aperture, on the other hand, is a bit more complicated to control, but is easy to understand. The aperture or iris simply controls how much light gets into your camcorder. You can manually control aperture settings, but the shutter speed and the amount of light in a scene will always be an influence on these settings as well.

Let's begin by examining focal length. The focal length of your lens directly affects depth of field. When shooting a wide angle shot, the focal length of the lens is relatively short and will have a deeper depth of field. If you zoom in, the focal length gets longer, and the shot will have a more shallow depth of field (Figure 2). If you want your subject to stand out in a crowded scene, you could use a long focal length lens, thereby decreasing the depth of field and making your subject the only area of focus. A lens with a short focal length is typically called "wide-angle" and a long focal-length lens is known as a "telephoto." In other words, when you are zoomed in, your lens has a longer focal length and therefore has a more shallow depth of field.

Subject distance from the lens and from the background also plays a part in depth of field (Figure 3). For example, decrease the distance between the camera and your subject and more of your shot will be in focus. To have only your subject in focus, increase the distance between your subject and the background, allowing the background to fall out of the depth of field.

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