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.

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