Home Made Collimator

A collimator will create a parallel beam of light from a point source. A lens focussed on infinity does the exact opposite so, to check if a lens is focussing accurately on infinity, it's possible to setup a collimator in front of the lens/camera and check the image formed at the focal plane to see if the beam is focussed back to a point light source. In reality, a point light source isn't used. Instead, a test target with a finely engraved mark is illuminated from behind and the same mark should appear sharpley focussed at the focal plane of the lens being tested.

I wouldn't say a collimator is essential if you are repairing lenses (others would disagree) but the only other way of checking if a lens is focussed on infinity is to aim it at a target a long way away and check the focus of the image at the focal plane.This is less precise but is probably accurate enough for most purposes. The other advantage of a collimator though, besides accuracy, is you can use it anytime, anywhere. So I decided to make one.

All that's necessary is a target image with diffuse illumination and a known good lens focussed on infinity. Probably the biggest difficulty if making a collimator from scratch is getting the lens accurately aligned with the target. The lens must be placed at precisely the correct distance from the target and the lens axis must be perfectly perpendicular to the target. To build a frame of some sort to do this would be difficult so I thought the best way to achieve it was to use and old camera body with a suitable lens attached. If the lens is made for the camera then we know the distance from the lens to the focal plane will be correct and we also know the lens axis will be correctly aligned so it makes the whole process very easy.

I opted to use and old Contax 139 body which had been stripped for spares. It still had the mirror and shutter assemblies in place but these were removed so there was no obstructions behind the lens. A test target was made by taking an old focussing screen and cutting a fine cross in it with the tip of a knife. The screen was then stuck to the film rails in the back of the camera making sure the diffuse side of the screen was against the rails and nearest the lens.

In use, the collimator (note: the picture here only shows the camera body without the known good lens attached) is placed on top of a small slide viewing screen which gives the desired diffuse light source. The known good lens is then focussed at infinity (good idea to put a bit of tape on the focus ring to stop it from moving).

The camera with the lens being tested has another spare focussing screen taped into the back against the film rails.

The camera/lens is the placed, lens down, on top of the collimator. The screen in the back of the test camera is checked with a lupe to see if the target is accurately focussed or not. (Note the Cokin filter holder used to create a flat surface for the test lens to be placed down on).

The split image section of the focussing screen, being used to check focus, is ideal as, with this lined up with the target it allows for very accurate checking of the focus.

Correct and incorrect.

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