Lens Bench - Continued
The observing microscope provides the high magnification that is required to observe and study any
image formed by the lens being tested, and is provided with a focusing adjustment.
The optical axes of the microscope and the collimator must be alined accurately and pass through the
geometrical center of the chuck on the nodal slide. In addition, the ways on which the microscope
slides, must be sufficiently precise so that this alinement will not be affected by any change in its
location along the ways. The alinement of collimator can be corrected by adjusting screws.
Uses. The lens bench is used to test lenses for the following:
Equivalent focal length. The lens to be tested is mounted in the chuck of the nodal slide, using a light
clamping pressure in order to avoid straining the lens, and the azimuth of the chuck is adjusted to 0. A
mark, with soft wax crayon, is placed at the center of the surface of the lens facing the microscope. The
microscope is focused on the mark and the chuck is oscillated with the lens through a few degrees of
azimuth on each side of 0. This oscillation will probably cause the image of the crayon in the
microscope to shift laterally. Next, the chuck is moved longitudinally along its own slide, the microscope
is refocused, and chuck is oscillated again. The process is repeated until a position of the lens and
chuck is found which will permit the image to remain stationary while the chuck with the lens is being
oscillated. When this location has been found, the image of the crayon mark is focused as sharply as
possible and the location of microscope on the ways of the bench is read by means of the vernier and
scale. This reading is often known as the constant of the lens bench, because it is the same for any
lens tested on it.
The microscope must not be moved by its own focusing adjustment. Move the microscope
support stand along the ways of the lens bench when focusing the microscope.
Move the microscope along the ways until it is focused on the image of the target. This setting should
be corrected by oscillating the lens in the chuck, and moving the chuck and microscope in their
respective ways in the same manner used previously, until the image of the target is stationary. When
proper position of the chuck and the microscope is determined, the new location of the microscope on
its ways is read on the scale, again using the vernier. The difference between this reading and the
constant that was previously obtained is the equivalent focal length of the lens.