f/stops have three functions:
1. They act as a partial control of exposure (the
other exposure control is the shutter).
2. They help control depth of field.
3. They allow the photographer to adjust the
aperture to the point of best definition of the lens,
sometimes called the optimum or critical aperture.
Each of these functions is discussed in this chapter.
A lens, at a given focus setting, provides a sharp
image of an object at only one distance in front of it.
However, when the distance between the focal plane and
the lens can be adjusted, the lens can be made to form
sharp images of objects located at differing distances in
front of it. Therefore, to get a sharp image of a subject
at a given distance, you must adjust the lens to the
appropriate distance from the film plane. This
adjustment is known as focusing.
In focusing a camera lens, the nearer the subject is
to the lens, the farther behind the lens the image is
formed. For close subjects, the lens must be moved away
from the film plane to focus the image; and the farther
away the subject is from the lens, the closer to the lens
the film plane must be (fig. 1-25).
INFINITY FOCUS.When the lens is focused on
an object so distant that the light rays reflected from it
are parallel, these rays converge (after refraction by the
lens) at the point of principal focus. The point of
principal focus is on the principal focal plane; that is, at
a distance of one focal length behind the lens. Therefore,
the lens is said to be on infinity focus.
When the distant object is moved nearer to the lens
or the lens is moved closer to the object, the distance
between the focal plane and the lens must be increased
to keep the image in sharp focus. When the distance
between the lens and focal plane is not extended as the
object is moved nearer to the lens, the image of the
object becomes blurred or out of focus. The closer the
lens is to the object it is focused upon, the larger the
image becomes until the distance between the lens and
the focal plane is extended to twice the focal length of
the lens. At this distance, the image and the object
focused upon are the same size. Therefore, the size of
an image formed by a lens is dependent upon two
factors: the distance from the lens to the object focused
upon and the focal length of the lens.
Figure 1-25.Subject distance and focus.