A quickie.....

Discussion in 'Large Format Cameras and Accessories' started by scootermm, Aug 6, 2004.

  1. scootermm

    scootermm Subscriber

    Messages:
    1,865
    Joined:
    Jun 10, 2004
    Location:
    Austin, TX
    Shooter:
    ULarge Format
    this will likely be a quick answer but Im gonna ask it even though it may be something basic that I should already know the answer too.....

    why are the corners clipped on groundglass?
     
  2. jd callow

    jd callow Moderator Staff Member Moderator

    Messages:
    8,003
    Joined:
    Jan 31, 2003
    Location:
    Milan
    Shooter:
    Multi Format
    I was under the assumption that it was to allow for air flow.
     
  3. Silverpixels5

    Silverpixels5 Member

    Messages:
    594
    Joined:
    Mar 19, 2003
    Location:
    Houston, TX
    Shooter:
    Multi Format
    i believe its to check the coverage of your lens. It also makes extending the bellows easier.
     
  4. Flotsam

    Flotsam Member

    Messages:
    3,221
    Joined:
    Sep 30, 2002
    Location:
    S.E. New Yor
    To allow air to escape/enter when compressing/extending the bellows?
     
  5. scootermm

    scootermm Subscriber

    Messages:
    1,865
    Joined:
    Jun 10, 2004
    Location:
    Austin, TX
    Shooter:
    ULarge Format

    okay see this is what confuses me...... for some reason Ive found it harder to check for vignetting with the clipped corners. causes weird shadows from the clipped corner etc.... maybe Im doing something wrong.
     
  6. matt miller

    matt miller Subscriber

    Messages:
    829
    Joined:
    Feb 11, 2004
    Location:
    Iowa
    Shooter:
    Multi Format
  7. Steve Hamley

    Steve Hamley Member

    Messages:
    453
    Joined:
    Sep 7, 2002
    Location:
    Knoxville, T
    Shooter:
    Multi Format
    Clipped corners serve 2 functions. It lets air in and out when you move the standards with a lens mounted, and it allows you to see if there is mechanical vignetting from a lens.

    For example, if I mount a 110 SS XL on my 8x10, then look at the aperture through the clipped corners, I see an oval until I stop the lens down a bit. The aperture is oval and not round because at wide apertures, the rear lens cell barrel obscures part of the aperture. You don't want to take a picture this way, so you look through the clipped corners to determine at what f-stop setting you can see the entire aperture.

    Steve
     
  8. Jorge

    Jorge Inactive

    Messages:
    4,532
    Joined:
    Sep 6, 2002
    Shooter:
    Large Format
    You have to look from the front through the lens, if you can see all four corners you have no vignetting.
     
  9. matt miller

    matt miller Subscriber

    Messages:
    829
    Joined:
    Feb 11, 2004
    Location:
    Iowa
    Shooter:
    Multi Format
    It makes sense to me that this method would work without clipped corners as well. Is that correct?
     
  10. jimgalli

    jimgalli Subscriber

    Messages:
    3,571
    Joined:
    Sep 7, 2002
    Location:
    Tonopah Neva
    Shooter:
    ULarge Format
    I've been doing HABS/HAER work for the company I work for with "their" camera which is a Cambo with a 210, 150, and a 6.8 Grandagon 90. The 90 gets used for 100% of the interior things and is constantly strained to the maximum rise / shift etc. I'd be lost without the corner cut-outs to tell me if I've passed the edge of the image circle.
     
  11. Jorge

    Jorge Inactive

    Messages:
    4,532
    Joined:
    Sep 6, 2002
    Shooter:
    Large Format
    It would, but is a little bit harder to see. With the clipped corners there is enough contrast and change in light intensity that it is very simple to see if you are vignetting one or two of the corners.
     
  12. Jorge

    Jorge Inactive

    Messages:
    4,532
    Joined:
    Sep 6, 2002
    Shooter:
    Large Format
    Double post....go figure..
     
  13. acroell

    acroell Member

    Messages:
    88
    Joined:
    Jun 7, 2004
    Location:
    Freiburg, Ge
    Shooter:
    4x5 Format
    Yes that is true. Vignetting can be checked from the front or the back, but the clipped corners are needed for the latter.
     
  14. steve simmons

    steve simmons Inactive

    Messages:
    368
    Joined:
    Sep 25, 2002
    You should be able to look from the back through the cut out corners to the lens or from the front through the lens to the gg and see the corners to check for vignetting. Either way.

    steve simmons
     
  15. sanking

    sanking Member

    Messages:
    4,813
    Joined:
    Mar 26, 2003
    Location:
    Greenville,
    Shooter:
    Large Format

    Vignetting can be checked from the front or the back but in very critical cases it is best to do so from the back. It is true that if you look through the lens from the front and can see the corners you will have at least some light falling on the corners. However, there is a point at which you can see the corners through the front of the lens but still get some light fall off because the corner is not seeing the entire circle of the aperture of the lens, but an elliptical part of it.


    Looking through the back is more precise. If you see an elliptical shape through the aperture you are getting light on the corners but there will still be some fall off. What you need to see is a full circle through the aperture, or at least a full oval, and then you can be absolutely certain that there will be no light fall off.
     
    Last edited by a moderator: Aug 6, 2004
  16. scootermm

    scootermm Subscriber

    Messages:
    1,865
    Joined:
    Jun 10, 2004
    Location:
    Austin, TX
    Shooter:
    ULarge Format
    everyone thanks so much for the advice. I think Ive shot hundreds of sheets of film already (not usually plagued by much vignetting - luckily)
    Sanking yours and steves explanation were especially helpful, it makes alot more sense now.