(1) Position your head in front of the lens assembly (figure 14) and
(2) Readjust the light assemblies (para 25b) to produce an even
b. Determine exposure. The exposure required for the type of copy and
the image size used may be determined by the operator based upon experience.
Normally, however, an exposure meter must be used. When an exposure meter
is used, proceed as follows:
(1) Operate the exposure meter to measure the light from the material
to be copied, and to determine the exposure setting (f/stop and time) for
the light measured and for the film used.
(2) The exposure setting obtained [(1) above] is correct for objects
at infinity (a lens operating at a distance from the film equal to the lens
focal length). However, the lens used with the camera set operates at a
distance from the film much greater (due to the bellows extension) than its
12inch focal length. As a result, a corrected exposure setting must be
(3) Calculate the corrected exposure setting as follows:
(a) Measure the bellows extension (distance from the center of the
lens to the film plane).
(b) Use the following formula to calculate the corrected exposure
CES is the corrected exposure setting in seconds.
BE is the bellows extension distance in inches [(a) above].
FL is the focal length of the lens in inches.
EM is the exposure meter setting in seconds [(1) above].