 Shooter's
Workshop
Understanding
and Manipulating Depth of Field
by Brian Pogue
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.
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| 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.
Figure 2
Distance
Yourself
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.
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