Shimming ground glass / fresnel position

Discussion in 'Large Format Cameras and Accessories' started by iamthejeff, Feb 8, 2018.

  1. iamthejeff

    iamthejeff Member

    Messages:
    43
    Joined:
    Mar 30, 2015
    Shooter:
    35mm
    I have a Graphic View II 4x5 and while my shots have not been terribly blurry, they haven't been terribly sharp either. I decided to measure the difference between the ground glass and film plane with a film holder installed, and was shocked to see that the ground glass needed to be shimmed back by a full 1mm!

    After reading about the topic I found that some cameras require the fresnel lens to be in front of the ground glass (closest to the lens), whereas some cameras require the fresnel to be behind the GG (closest to the photographer). My camera was set up so that the fresnel was behind the lens, and the fresnel is roughly 1mm (maybe a bit more) in depth. I am wondering perhaps if the previous owner of my camera had the fresnel/GG installed incorrectly. Does it make sense that the ~1mm difference would be attributed to the fact that the fresnel should have been placed in front?

    I guess I have some test shots to make but I just wanted to ask first.
     
  2. ic-racer

    ic-racer Member

    Messages:
    8,424
    Joined:
    Feb 25, 2007
    Location:
    Midwest USA
    Shooter:
    Multi Format
    Could be.
    You can also photograph a ruler. Record the number at thich GG shows focus and then process the negatives and see the difference. You can solve this equation for "GG error" to see if any fine-tuning other than your 1 millimeter is needed. If you get the "+" or "-" units mixed up remember that if the GG is to far back the GG image focuses IN FRONT of the negative focus.

    1/difference + 1/GG error = 1/focal length
     
  3. shutterfinger

    shutterfinger Subscriber

    Messages:
    2,334
    Joined:
    Feb 25, 2013
    Location:
    San Francisco Peninsul
    Shooter:
    4x5 Format
    Graflex Corp. milled down the mount post that the ground glass sits on and placed the Ektalite Field Screen in front of the ground glass so that the Ektalite Field Screen is 1/2 the optical focal length of the field screen forward from the ground glass.
    Now if you have a factory original field screen then placing it in front of the ground glass with the ribbed side facing the ground glass then the ground glass etched side against the field screen should correct the focus error.
    If you have some other fresnel then it may not correct the focus error if it is correctly placed in front of the ground glass.
    Leaving the fresnel behind the ground glass shim the ground glass so that it is the same distance as the film plane in a film holder. Brass makes a good shim. 1mm=‪0.03937‬ inch. .040 brass should be readily available at hardware or model train/plane supply stores as is .020 and .035.
    Ektalite Field Screen is Graflex speak for fresnel.
    Field Screen backs have short mount post/studs for the sandwich, taller ones for gg only.
    low- .040 to .050, high .09 to .1 inch. (1mm low; 2.5 high)
     
    Last edited: Feb 8, 2018