Depth of Field – Selective Focus
Depth of Field Concept
We have all seen pictures in which everything from foreground to background is in sharp focus. In other pictures
only the main subject appears sharp, while its surroundings are a soft, out-of-focus wash of color and shape.
This effect is to due a concept known as “depth of field.”
The exposure required to capture a clear picture will depend on the light reflecting from the scene, and the
overall brightness of the subject. As the videographer, you do determine how much of that light passes through
the lens of your camera -- and for how long. There are always two factors involved in making your decision:
Shutter Speed
The shutter speed controls the amount of time the camera curtains (or the overlapping metal leaves in
some lenses) remain open. Common shutter speeds include: 1 second, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60,
1/125, 1/250, 1/500 and 1/1000 of a second. Obviously, the longer the shutter speed, the more
light will reach the film at any given aperture.
Aperture
The aperture is the size of the opening in the diaphragm
mechanism of the lens (see the illustration). The larger the
aperture, the more light there will be to expose the film
during any given time. The aperture settings on most lenses
(called f-stops), run in a series from the largest to smallest
opening. The higher the f-stop number, the smaller the lens
opening. Each f-stop is doubling (or cutting in half, depending on
which way you're going) the amount of light entering the lens.
Controlling Depth Of Field
Now, here's the important point: Depth of field is not constant. It can be varied, using a number of tricks.
DOF is basically determined by the iris setting. The smaller the iris aperture, the
greater the depth of field. Naturally, in very low-light situations where the iris is open wide,
depth of field is significantly reduced and focus becomes quite a challenge.
Factors which influence depth of field:
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Lighting conditions (bright light closes the iris and promotes “Great” DOP
where everything is in focus)
Shutter Speed (Faster speeds allow for a larger iris setting)
Lens angle (Wide angle vs. zoom) – Easier to control when zoomed in (smaller
focal range)
There are many reasons for wanting to alter the DOF. Perhaps the simplest is to make the
audience concentrate on the subject. If you want to reduce DOF, you can:
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Open the iris (PORTRAIT MODE *)
Increase the shutter speed.
Zoom-in and move the camera/tripod back from the subject.
* Many consumer camcorders have a feature known as "portrait effect". Activating this
feature reduces DOF by adding a little shutter, forcing the auto-iris to open wider. As you
might expect, you'll have more control if you select shutter speed and iris setting yourself.
http://www.andylim.com/photo/depthoffield.htm
http://www.mediacollege.com/video/camera/focus/depth-of-field.html