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Operating Instructions—C-30B Camera
Figure 2-13. Depth of field for different aperture settings.
EFFECTS OF CAMERA MAGNIFICATION
ON EXPOSURE
The camera or lens object-to-image ratio is the ratio of
the object size to the image size. For example, a 1:1 setting
is one whose object is the same size as the image. The
object-to-image ratio o f the camera lens has a definite effect
on the exposures obtained. It affects the amount of light
which w ill fall on a given point of the film . The smaller the
image on the film , the greater the intensity of the light.
Therefore, the larger the object-to-image ratio, the less time
is required to obtain a good exposure on the film . With a
1.07 setting, slightly less exposure time is required to
Photograph a display than with a 1.09 setting. The
difference in the exposure time required between the
various settings is so siight, however, that it can usually be
ignored.
It is important to note that a better writing rate can be
obtained with a small object-to-image ratio than with a
large object-to-image ratio. Here again, the difference is
fairly small unless there is a wide difference in the ratio
(e.g., 1.05 has a better w riting rate than 1.2).
RECIPROCITY
Decreasing the f-STOP setting by one unit (e.g., from f/8
to f/5.6) doubles the area through which light can pass and
expose film in the camera. Theoretically such a decrease in
f-STOP requires that the exposure time be halved to pro-
duce the same exposure. This results in what is known as
the Law of Reciprocity. This law works quite well for
medium-intensity light at medium shutter speeds. It fails,
however, fo r very short exposure times taken with bright
light or fo r very long exposures from dim light. The
Reciprocity Law applies only for exposures in the ränge of
approximately 1/250 second to 1 second.
2-17
Summary of Contents for C-30B
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