Ground glass and film plane

Discussion in 'Camera Building, Repairs & Modification' started by RobertP, Feb 14, 2009.

  1. RobertP

    RobertP Subscriber

    Messages:
    1,130
    Joined:
    May 11, 2006
    Shooter:
    ULarge Format
    I just had someone tell me that if I would turn my ground glass around where the frosted/ground part is facing me and not the lens then that will change my plane of focus. I had a camera built that is designed with the GG that slides in and then is removed, The builder installed the groung glass with the frosted part facing away from the lens. He then set the film plane to match the ground side of the ground glass which is now facing me. He claims the film plane should match the ground side of the glass which is true if the ground side is facing the lens like it should. So my question is by just turning the ground glass around does that change the film plane. Or is the film plane the same no matter which side of the ground glass is facing the lens? Thanks, Robert
     
  2. E76

    E76 Member

    Messages:
    373
    Joined:
    Mar 20, 2008
    Location:
    Rochester, N
    Shooter:
    Instant Films
    Turning the ground glass around will change the plane of focus, because it is on the ground side of the glass where the image actually appears.
     
  3. RobertP

    RobertP Subscriber

    Messages:
    1,130
    Joined:
    May 11, 2006
    Shooter:
    ULarge Format
    You're absolutely right. I got out the tape measure and did some focusing and measuring and the image does focus on the ground side of the glass. I never gave this any thought until someone set my GG up in reverse. Thanks
     
  4. freygr

    freygr Member

    Messages:
    137
    Joined:
    Jun 5, 2007
    Location:
    Portland Ore
    Shooter:
    Multi Format
    There is a problem with the ground glass out, the refraction of the glass will throw the focus off, but not by much. It may depending on how fast the lens is and the focal length, it may not mount to a hill of beans with long and slow lenses.
     
  5. goch@unb.ca

    goch@unb.ca Member

    Messages:
    2
    Joined:
    Mar 5, 2009
    Shooter:
    Multi Format
    Hmmm. What was the reason for "reversing" the ground glass? I would think that you would want the gg *in* to protect it from getting marked up. Are you using a fresnel lens on top of the glass? Then the reversal could make sense.
     
  6. Stefan Findel

    Stefan Findel Member

    Messages:
    80
    Joined:
    Feb 15, 2008
    Location:
    Rhinebeck, N
    Shooter:
    Large Format
    No, it does not! You would still have wrong focus! Your film emulsion would end up closer to the lens than the focus plane of the GG, thus the focal plane would be further away than where you had focused. This would possibly ruin shots, where focus is critical as in stillife, close portraits, use of long lenses, open apertures, to name a few.