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Thread: Testing for Correct Position of Ground Glass

  1. #1

    Join Date
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    Testing for Correct Position of Ground Glass

    I just installed a Maxwell screen to replace the Linhof ground glass in my Master Technika (which in turn replaced the Beattie screen that was on the camera when I bought it). Since this is the third screen that's been on the camera I'd like to to make sure the position of the screen is correct. I remember an article in View Camera magazine some time ago that described a testing methodology using playing cards and I vaguely recall another method mentioned here of focusing on a point on a ruler. However, I don't remember the details of either method. Would someone please describe a method they've used to test the correct positioning of the ground glass or cite me to a place where a detailed description can be found? I searched the archives but unfortunately the word "test" or "focus" appears in hundreds of messages. and there doesn't seem to be an "exact phrase" method of searching the archives. Thanks.
    Brian Ellis
    Before you criticize someone, walk a mile in their shoes. That way when you do criticize them you'll be
    a mile away and you'll have their shoes.

  2. #2

    Testing for Correct Position of Ground Glass

    I can't help you with the proceedure you mention but I can tell you how I do it on the 4x5 cameras I build.

    The distance from the face of the film holder to the surface of the film is 0.197" (5.00mm) on a 4x5 holder. I check the position of my ground glass by measuring from the face of the GG frame to the surface of the glass and make sure it is exactly 0.197" If the distances are the same, they will both focus the same. (Dial caliper or depth micrometer will both work.)

    I do not know if the distance is the same with other film formats.

  3. #3

    Testing for Correct Position of Ground Glass



    Here are two previous discussions from the archives: Ground Glass focusing errors at http://www.largeformatphotography.info/lfforum/topic/118997.html and
    Is my ground glass mis-seated? at http://www.largeformatphotography.info/lfforum/topic/497848.html.


  4. #4

    Join Date
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    Testing for Correct Position of Ground Glass

    Hi Brian,

    Put a ruler a foot or so in front of your lens, at an angle to it (exact angle doesn't matter). Focus with the ground glass on one specific line on the ruler, and take a shot with the lens wide open. Negative should be sharp at that line, or if not you can tell from which line is sharp as to whether the GG is too near or far from the focus plane. I have found that a Polaroid is good enough for this test.
    Wilhelm (Sarasota)

  5. #5

    Join Date
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    Testing for Correct Position of Ground Glass

    When you replace the gg on a Master Technika you simply place it on top of the shims. You do NOT adjust the shims. Therefore there should be nothing necessary for you to check. Unless someone has adjusted the shim needlessly.

  6. #6

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    Testing for Correct Position of Ground Glass

    Bob, my concern was that someone before me did something to the shims or did something else that might create a problem. The camera had a Beattie screen on it when I bought it, which means someone before me did at least one replacement of the original screen, and I don't know what they did or how many different screens have been on the camera. I suspect everything is fine but out of what might be an excess of caution I thought it would be best to test to be sure. Thanks for your respoonse and those of the others who responded.

    Calamity, I couldn't measure something to a distance of 0.197 inches if my life depended on it. I have trouble figuring out which line is a fourth of an inch and which is an eighth of an inch on a ruler.
    Brian Ellis
    Before you criticize someone, walk a mile in their shoes. That way when you do criticize them you'll be
    a mile away and you'll have their shoes.

  7. #7
    Photographer, Machinist, etc. Jeffrey Sipress's Avatar
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    Testing for Correct Position of Ground Glass

    If a camera is built to precision standards, why would shims be neccessary?

  8. #8

    Testing for Correct Position of Ground Glass

    Reply to Jeffery: I don't know about the camera in question, I used to rebuild heavy duty transmissions. In most trannys there was a bore for a bearing. On the german trannys, it was bored a little deaper so it could be shimmed to perfect specs. When doing production machining there will be a tolarence. I suspect it was machined with shimming to spec in mind. Just my guess, but bob would be the one to know. John

  9. #9

    Testing for Correct Position of Ground Glass

    Brian, the late Barry Thornton had avery good chapter on testing in his book "elements of darkness". Basically you can pick something like a picket fence, identify a single slat and focus on that slat, take a pic (polaroid is great for this) and then look for the slat in focus. If the one you chose is the one you picked then your GG is aligned, if the one in focus is closer to you and away from the "central" slat, then your gg is too far out, if the one in focus is farther from you and the "central" slat then the gg is too close to the frame.

    Of course in his book he built a special target for this, but you can do it as I describe, all you have to do is pick something that has a row of uniform things in sequence. I guess any fence will do.

    Good luck.

  10. #10

    Join Date
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    Testing for Correct Position of Ground Glass

    Brian,

    A few comments.

    It is best to have the ruler or other target making a 45 degree angle with the lens axis. I find a sheet of newsprint taped to a wall and photographed about 5 feet from the lens is a good choice of target.

    You are never going to get it exactly right because there are inevitable focusing errors. So you should do it several times to be sure.

    There are some simple formulas which can be used to determine the amount of the shift from gg to film plane. Suppose the position in the subject seems to shift by a certain distance along the target. Take 70 percent of that shift to account for the 45 degree angle. Then the actual shift on the film side of the lens is that amount multiplied by the square of the magnification. The magnification can be obtained in a variety of ways. One is to take the subject to lens distance, divide by the focal length, subtract one, and then take the reciprocal of the result. For example, suppose the camera is focused on the target 1.5 meters or 1500 mm from a 150 mm lens. 1500/150 = 10. Subtract 1 to get 9. The magnification is 1/9. Suppose there is an apparent shift along the target of 2 cm or 20 mm. Multiply that by .7 to get 14. The shift in the film plane is 14 x (1/9)^2 = 14/81 ~ 0.17 mm.

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