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Thread: Critical height placement of ground glass

  1. #11

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    Jan 2019
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    Re: Critical height placement of ground glass

    Quote Originally Posted by grat View Post
    Pardon my ignorance, but isn't the critical factor the location of the film plane? The film, and the ground glass, need to be on exactly the same plane for focus to remain consistent when you insert the film holder(s). How thick is the glass on the one that's raised versus the one that isn't? Does it have consistent focus when you take pictures? If so, that's what's relevant.

    As for the sloped corners, it would allow precise control over the depth of the ground glass by trimming the corners, if you couldn't get glass at a consistent thickness, and would eliminate the need for any type of shim.
    Yes they should match, ideally. You focus to the ground glass and then put the film at that location. If you focus wrong because the ground glass is not aligned (e.g. you put a fresnel between the ground glass and the lens) or because the holders have issues (OP's problem) then you will turn a sharp dot into a circle, which is the issue of depth of focus.

  2. #12

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    Jan 2019
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    Re: Critical height placement of ground glass

    Quote Originally Posted by Bob Salomon View Post
    This is what you said:

    “. . . Depth of focus is given by the nearest and furthest points of apparent sharp focus in front of the lens.. . .”
    And you're right that I wrote it wrong. My apologies. The math is still correct for OP's case, but that sentence explains depth of field, not focus, which is what I was trying to explain :-)

  3. #13

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    Re: Critical height placement of ground glass

    The ground glass needs to sit at the same distance as the film from the back of the camera. Measure the depth of your holders with film, then the thickness of the frame where the ground glass sits. In other words, the frame thickness has to equal the depth of your holders. That will tell you if it is correct or not. You will obviously need calipers to do this. If the rail on the frame is correct then figure out where the rail gets thicker and clip the ground glass before it hits that point.

    While you are at it, you might at well check to see if all your holders are the same...

  4. #14

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    Re: Critical height placement of ground glass

    If one is interested in producing in focus images on film based on the focused image on the ground glass, the diffusion side of the ground glass MUST be at the same mechanical dimension as the film in the film holder. This means the tolerances between diffusion side of the ground glass to film in film holder within thousands of an inch if properly done. The belief stopping down will fix it is not a good assumption as that is not the proper cure to achieve precise focus register to ground glass diffusion side to film sheet.

    This requirement becomes more demanding with large aperture lenses and increasing sheet film size, a design requirement that is difficult for lightweight wood folding cameras to meet.


    Bernice

  5. #15

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    Sep 2008
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    Sarnia, Ontario, Canada
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    826

    Re: Critical height placement of ground glass

    When I requested Richard Ritter to make replacement dark slides for my two 8x20 holders, he requested that I sent the camera back with them, as well.

    As it turned out, Richard needed to shim the ground glass, as it was in the wrong position. His explanation was that when my camera was built (late 1920s, early 1930s), it was common practice to put the ground side of the GG on the outside of the camera, rather than on the inside (i.e. between the back and the lens).

    Than could account for a significant difference, if that is the situation in your case.

    Just hope I correctly interpreted Richard;s explanation...

  6. #16

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    Nov 2017
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    Whidbey Island WA
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    Re: Critical height placement of ground glass

    Thanks for all the great responses everyone! Looks like I have a little bit of discovery and work to do.

  7. #17
    Nodda Duma's Avatar
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    Nov 2014
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    Re: Critical height placement of ground glass

    Gonna skip all the math and just say you should place the ground glass within +/- .005” of the film plane in a loaded holder relative to a common reference surface such as the lens board.
    Newly made large format dry plates available! Look:
    https://www.pictoriographica.com

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