trammel rod approaches the focusing glass, move its end in
across the entire surface, or trammeling the surface until all four
small circles until it lightly touches the glass, when the assistant
corner measurements are equal.
immediately stops the drive. Measure the trammel rod length,
b. Lensboard Settings. To position the counter so
and add to it the difference between the equivalent and back
that it will provide correct lensboard positions in following the
focal lengths of the lens. Then set the lens counter (3, fig. 1-4)
readings on the focusing chart, it is only necessary to determine
to read this distance in one one hundredth of an inch. (see para
accurately the exact distance between the vertex of the back
surface of the lens and the adjacent surface of the focusing
glass. The counter reading will then be this distance plus the
Note
difference between the equivalent and the back focal lengths of
The lensboard counter is set to agree with the principal
the lens. To measure this distance accurately, use a trammel
lens used (the Goerz 19 inch lens). The focusing chart
rod of known length (such as 48.000 in.) or one with a length
prepared for the second lens is first computed and then
which can be measured with a precision caliper or micrometer.
has a lens constant added or subtracted from each lens
Place a sheet of lens tissue over the back of the lens, attaching
reading. This constant is a measurement of the
it to the lens plate with a tab of self-adhesive tape. Then detach
difference in location of the back nodal points between
the bellows (7, fig. 4-18) by loosening clamp and screw (6) on
the principal and secondary lens. Accordingly, if the
the camera back, and strap them to the lensboard (1) with strap
camera must be calibrated to the secondary lens, it will
(8) so as to provide access for the measurement. Move the
be necessary to add or subtract the lens constant
lensboard away from the focusing glass to a distance of one-
(depending on whether it is plus or minus, that is,
half of an inch or so greater than the trammel rod length. Then
whether the rear nodal point is a greater or lesser
very gently place the trammel rod against the lens tissue and in
distance from the mounting surface on the lensboard) as
contact with the vertex of the lens, holding the end of the rod
can be determined from the lens data appearing on the
lightly against the lens (fig. 5-1).
With the other end of the
focusing charts. As the lens tissue normally used is
trammel rod supported by the other hand, have an assistant
approximately 0.001 inch thick, it is not necessary to
turn the lensboard fine adjustment handwheel (7, fig. 1-4)
allow for this in setting the counters.
slowly to bring the lens toward the focusing glass. As the
Figure 5-1. Tramelling focusing glass and lens.
5-4
