LESSON 2
(3) A
bellows should be capable of extending to at least twice the
focal length
of the lens. This amount of bellows extension is necessary to
produce 1:1
(same size) reproductions.
The bellows also plays a role in
focusing the
image on paper.
(4) A lens. The focal length of the lens should
cover the angle of
field of the negative being printed. A general rule of
thumb is to use a
lens whose focal length equals or exceeds the diagonal
measurement of the
negative being printed. The following is an example of
the proper lens to
be used with the three most common negative sizes.
(a) 35 mm negative uses a 50 mm lens.
(b) 120 negative uses a 105 mm lens.
(c) 4- x 5-inch negatives require a 150 mm lens.
(5) An easel.
There are many types of easels in use, each serving
the same basic purpose; to hold the printing paper in a flat plane.
c. There are, basically, two types of projection printers,
the
condenser type and the diffusion type, as shown in Figures 2-5 and 2-6.
(1) The condenser type projection printer (fig 2-5) has a set of
condensing lenses between the printing light and the negative.
These
condensing lenses align and project the light rays evenly through the
negative. The condenser type printer produces more contrast and a sharper
image from a negative, than the diffusion type printer. As a result, any
defect in the negative (dust, lint, scratches, etc.) are faithfully
reproduced. In addition, scars, acne, wrinkles, etc. will be very sharp and
readily noticeable in the print.
(a) Condenser lenses are designed and ground to provide maximum
image sharpness when used with a given focal length lens. Therefore, if you
change projector lenses, you must also re-position the condensing lens.
(b) The proper position for use with the different
lenses
is
indicated on the inside flap of the variable-condenser housing.
(2) The diffusion type projection printer (fig 3-5) has a diffusing
medium (usually translucent glass) between the light source and the negative
to spread the light evenly over the entire surface of the negative. Light
emitted from the lamp, as well as that reflected from a parabolic reflector,
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