How does this work? using a second camera for collimating

Here's a link to a Mike Elek page showing how to use a second camera with a long lens to collimate the lens on another camera: http://elekm.net/zeiss-ikon/repair/collimate/

Both lenses set to infinity, distance between them is immaterial.

I can't get my mind wrapped around how this works optically. Has it to do with virtual focal points/nodes? Maybe there is a longer more detailed explanation you can point to?

Thanks,
Keith

A collimated beam is a parallel bundle of rays. Light from an point source very far away is a very slowly diverging cone, and the limiting case of this is an object infinitely far away, where the light rays coming from it are parallel to each other. Of course, nothing is infinitely far away, but something that is many hundreds of focal lengths is close enough.

If you focus your camera at infinity, it's collecting light from parallel rays entering the lens and focusing it onto a single point on the focusing screen (and the film, if your camera is adjusted correctly). In Mike's setup, he reverses it, sending light from the target at the film gate through the lens and into the SLR used to observe the ray bundle.

The assumption is that the SLR focusing screen and SLR lens agree on infinity focus - since it's a SLR, this is easy to check, just focus on a far away object. Then for the camera to be calibrated, which is a Contax RF in Mike's pictures, he puts a target at the film gate. Light from the target passes through the RF lens. If that lens is set correctly at infinity, the light will be collimated, enter the SLR lens, and be focused correctly on the SLR screen. If the RF lens is not at infinity, then he adjusts the RF lens so that it agrees with its own focusing scale.

Thanks much!

OK, so... when the light from the target passes through the RF lens most of the light rays are made to be approx parallel if that lens is correctly set to infinity focus, which is the reverse action we normally use the lens for- normally we use the lens focused at infinity to gather light rays which are approx in parallel to focus them at a single point on the film plane. So... by projecting the light through the film plane of the RF through the RF lens we form the rays into parallel lines which then enter the lens of the SLR still as parallel lines... and that is why the distance between the two lenses is not (within reason) a limiting factor. Further, since we have the SLR set to infinity focus we will only see the light rays from the RF as in focus is they are bundled parallel rays- which is what collimating means.

I think I have it... Thanks again!