Figure 1-28.Depth of field.
When a lens is focused on an object at a certain
distance, other objects, both closer and farther than the
focus distance, form larger circles of confusion. When
the film is placed at a point corresponding to the lens
focus distance, a clear image is produced (fig. 1-28).
When the film is nearer or farther away from the lens
than the corresponding lens focus distance, the image
becomes blurred because of the larger circles of
confusion caused by the intersection of light rays either
in front of, or behind, the film plane.
Table 1-3.Permissible Circle of Confusion Is Dependent on
2 1/4 x 2 3/4"
4 x 5"
8 x 10"
Another factor affecting the circle of confusion is
lens aperture. Decreasing a lens opening narrows the
light rays passed by the lens. The narrower these rays,
the smaller the circles of confusion when the image is
not in perfect focus. In practice, this means that a small
lens opening is used to record, as clearly as possible,
several objects at varying distances. Even when the rays
from some objects do not intersect perfectly at the film
plane, the circles of confusion ahead or behind the film
are negligible and still appear as a sharp image.
The size of the permissible circle of confusion
depends on the film format size and the manner in which
the film will be used. Experience has shown that the
permissible circle of confusion should not exceed about
1/1000 of the focal length of the lens. This is normal for
the film size. The generally accepted permissible circle
of confusion diameters are given in table 1-3.
Figure 1-29.Depth of focus.
The minimum circle of confusion of most lenses is
sharp. Consequently, the distance that the focal plane
small. Thus the focal plane can be moved slightly and
can be moved forward or backward from the plane of
yet retain an acceptable sharp image. However, as the
sharp focus and continue to produce an image of
distance of the movement is increased, the circle of
acceptable sharpness is termed the depth of focus. This
confusion becomes greater and the image becomes less
depth is always within the camera (fig. 1-29).