Re: Draft: Matching....

[T3D Sam Smith <3dhacker@xxxxxxxxxxxxx>]

Great article from John Ohrt on lens matching, but he left out one of my
favorite techniques.

How come you left out the "third" method of precision focusing, the
collimator? I have found the surface of most ground glass much too coarse to
accurately tell precise focus, especially the "miniature" formats.

The collimator method gives a very precise reading of infinity. A front
surface mirror is placed at the film plane, bouncing the light through an
objective and back to a target at the source. In between is a two way mirror
that reflects part of the image up to an ocular fitted with a crosshair. When
the image is in perfect focus with the crosshair, you have infinity focus.

This method is limited to the center area at infinity, but it does help
eliminate any doubt as to how well at least part of the film plane is
focusing. Perhaps with a little adapting the same method could be used for
edge focus as well as different focal distances, but that's beyond me at
this point.

Sam


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