If you're searching for ways to vary the look and feel of your video images, one of the easiest compositional elements to manipulate is depth of field.
Depth of field is a term used to describe how much of your shot is in focus, front to back. An image that has a narrow depth of field will have a sharp focus on a particular object, with objects in the background and foreground falling out of focus (Figure 1a). If an image has a wide depth of field, all objects, both foreground and background, are crisp and clear (Figure 1b). Artistically, it isn't always desirable to have the entire scene in focus.
The depth of field of your images depends largely on your circumstances. If you have a background that is distracting, you may want to throw it out of focus to de-emphasize it. A blurry background is a great way to keep your viewer's attention on your subject. If, on the other hand, you want your viewer to see all the surroundings in a scene, a greater depth of field may be appropriate.
Figure 1a Figure 1b
Manipulating Depth of Field
While depth of field is ultimately an optical characteristic that depends primarily on the size of the aperture (iris, f-stop) and the focal length of the lens, the techniques you use to get the right shot can be tricky. The wider the aperture setting, the shallower the depth of field ( Figure 2). Think about it, when you shoot outdoors on a bright day, everything is in focus. When you shoot in low light, it can be hard to get anything sharp.
One of the simplest and most effective ways to vary your depth of field is by changing your camera-to-subject distance. This simply means picking up the camera and moving it closer to or further away from your subject and then reframing the shot using your camera's zoom. When you move your camera back, you will need to zoom in to keep your subject properly framed, for example. A telephoto lens setting will give you a longer focal length and a more shallow depth of field. A telephoto setting allows less light to hit the CCD, which narrows your depth of field. A wide-angle setting means a short focal length and a wide depth of field. So, if you wanted to narrow the depth of field to throw the background out of focus, one way would be to move the camera farther away and zoom in on your subject.
Let's say you are taking footage of a vase sitting on a table, and that the vase is positioned ten feet away from your background. If you place the camera one foot away from the vase, you will need to have your lens at a fairly wide setting in order to have the vase properly framed. Because of this wide-angle setting, your depth of field will be deep enough so that both your vase and your background are in focus.
If, however, you want to de-emphasize the background, you can accomplish this by moving the camera farther away from the vase. Instead of one foot from the subject, move the camera back ten feet away and zoom in to frame the vase. Now you'll be able to throw the background out of focus more easily. Keep in mind that the greater the distance between your subject and the background, the more flexibility you have.
It is difficult to obtain varying depths of field if there is no actual depth between your subject and background. Small studios and interior shots may physically limit your creative choices, but even subtle changes can make a big difference.
Brian Pogue is a professional video producer and an educator.