Collimators and Auto Collimators
There is a bit of confusion in terms. A collimator is a tube with a lens at
one end and a reticule at the other. The reticule is focused at infinity so
that when a lens system or camera looks into the collimator it sees an image
at infinity. These are useful for checking the infinity focus marks on lenses.
They can also check to see if a zoom "holds focus". Some sharpness evaluations
can also be made if the reticule is fine enough. Collimators with target reticules
are used in aligning machinery, binoculars, and other optical instruments. Collimators
are available surplus and can be used to check infinity focus. If a light source
is not supplied behind the reticule, add one.
Autocollimators are a collimator with a reflex viewing system and a light source
that projects the reticule infinity image on the film or a mirror at the film
plane. That image is then viewed back through the lens by the eye via a beam-splitter.
Smaller units can be attached as a filter sized disk on a lens. The image is
quite dim reflected off the ground glass, but can be viewed through the eyepiece
to check correct position of the ground glass. Larger units need to be used
on an optical bench or matte box rods where the autocollimator and camera are
easily aligned. The size of the reticule image seen in the autocollimator does
not chance with the different focal lengths. It does change on the ground glass.
A right angle prism with a ground surface can be cut to fit many camera aperture
plates. This is a field check to check if a film plane or ground glass is out
of position. If you are shooting in distant locations far from service, you
might consider this option. Check the lens marks with focus on the ground prism
against focus in the viewfinder. If you have a problem with the viewfinder,
trust the lens focus marks. If the viewfinder and lens focus marks agree, but
image on the ground prism doesn't, you have a problem with the position of the
aperture plate. If the viewfinder and ground prism both agree, you might have
a lens mount problem. Check if all the lenses agree. Remember that a lens mount
seating problem will be worse the wider the lens where focus is harder to see.
You can grind a prism with another piece of glass and some course glass-grinding
compound. Be careful to not scratch the aperture plate or damage registration
or pull down pins. Arri II and III models are fairly easy to make. Do make a
device to hold the prism. Use a 50-mm lens to look at the image. An old still
lens is better than a $2,000. Prime.
© Copyright 1999-2004 Ron Dexter. All Rights Reserved.