Proper way to shim lens mount?

Discussion in 'Camera Building, Repairs & Modification' started by Yashinoff, Nov 8, 2012.

  1. Yashinoff

    Yashinoff Member

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    I recently disassembled an Edixa SLR to reglue a shutter strap that came loose. This required removing the mirror box and consequentially the lens mount from the camera body. The mount had a number of shims behind it to align it properly with the film plane. I want to know what the proper procedure is for checking the alignment of the mount with the film plane. I assume this is something that cannot be done by simply checking focus at the film plane with piece of ground glass, or can it? I doubt one could see a tiny amount of misalignment this way.
     
  2. E. von Hoegh

    E. von Hoegh Member

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    Without knowing (seeing) where the shims were, it's hard to say. I'll assume the focussing screen is fixed and not shimmed. You need to place the mount flange surface a specific distance from the filmplane as well as perfectly square to the film plane. I'd use a surface plate, gauge blocks or precision parallels to rest the camera on, and a good dial indicator reading to .0001". Focus would be checked by viewing the aerial image at the plane,(both film and GG) not on groundglass. Next time record where the shims were and replace them precisely as they were.
    Good luck.
     
  3. Nicholas Lindan

    Nicholas Lindan Advertiser Advertiser

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    A depth micrometer is what you want.

    Place a sheet of glass (a microscope slide works) or something else very flat against the film rails and measure from the mounting surface of the lens mount and the front surface of the glass/'something else'. It is called the "film to flange" distance.

    The correct distance for an Edixa is 53.0 mm / 2.087".
     
  4. Yashinoff

    Yashinoff Member

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    The mount is held onto the camera body by four screws, each of which has a number of shims behind it. Unfortunately a few fell out when I removed the mount from the lens throat. I'd figure it would be best to check the alignment anyway if I had bothered to count them.

    The focus screen is also shimmed, I will check it against the film plane once I have the mount alignment set. Thank you for the suggestions.
     
  5. Mark Crabtree

    Mark Crabtree Member

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    Fussy work, but doable. Just thinking aloud - Once you get it close, maybe a test shot with a sharp lens (shorter rather than longer FL - normal should do) of a distant view to check sharpness across the frame in all directions. At close distances it will be hard to be certain the camera is absolutely parallel to the subject.
     
  6. E. von Hoegh

    E. von Hoegh Member

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    A depth mike will not give an indication of whether or not the flange is square to the film plane. As I mentioned above, support the camera at the filmplane on a surface plate and use a dial indicator to measure paralellism. The depth mike will be good for measuring the correct distance from flange to plane.

    To the op: if the shims for the screen weren't disturbed, leave them be for now. The first step is to get the flange parrallel to, and the correct distance from, the filmplane. Second step is to get focus on the the screen to coincide with the focus on the filmplane.