2-13. CIRCLE OF CONFUSION.
a. Basically, a photograph is an accumulation of many points that are
exact images of points composing a subject. For example, light rays striking a
subject are reflected by a lens, and are reproduced on film or ground glass as
circles.
b. Light rays seem to produce a "cone" of light.
The apex of the cone
originates at a point on the subject; the base of the cone is at the lens.
When the light rays pass through the lens, the cone of light is reversed. The
base is still at the lens, but the apex now lies in the focal plane.
An
infinite number of these cones combine to produce a photographic image.
c. If the cone of light is intersected, either in front or behind the
focal plane, the light rays form circles rather than points. These are called
circles of confusion, (fig 2-19).
A circle of confusion is a point of light
that has been enlarged due to incorrect focus.
It is measured as a fraction
such as 1/250, 1/500, or 1/1000 of an inch.
d. If the circles of confusion are small enough, they are
"acceptably"
sharp to the eye and are said to be in focus. If the circles are
large enough
they appear as circles and the image, now consisting of many
circles, is
blurred and out of focus.
It should be noted, however, that
a circle of
confusion may be acceptable for one photograph and unacceptable
for a second
type of photograph.
e. Factors Controlling Circles Of Confusion.
(1) Focusing. Light rays form sharp points on the ground glass or film
if the subject is at a correct distance from the lens in relation to the
distance between the lens and the film. At this point, the image is sharp and
the lens is focused properly, (fig 2-19).
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