a. The depth of field can be shallow or great. If you have a shallow
depth of field and you focus on an object in the middle ground, the
foreground and background will be out of focus. If people or objects move
around in a shallow depth of field, they quickly move out of the depth of
field and get out of focus, unless you adjust the focus on the camera. The
same thing may happen when you move the camera.
b. If the depth of field is great, all objects in the foreground,
middle ground, and background, will be in focus, even though you focused on
the middle ground object only. If you have a great depth of field there is
a large sharp zone in which people or objects can move around and stay in
focus.
This great depth of field makes it possible to dolly and stay in
focus. The depth of field depends on the coordination of three factors:
(1) The focal length of the lens.
(2) The aperture.
(3) The distance of the object from the camera (fig 2-3).
Figure 2-3.
Depth of field
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