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Thread: focal plane of ground glass not in sync with film

  1. #1

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    focal plane of ground glass not in sync with film

    Does anybody have a method of 'recalibrating' or finding the correct distance of where the ground glass focuses to where the film actually lies with-on the film holder back? I need to replace the ground glass on this Raja I just picked up, but anyways with the ground glass on it, the pictures came out really blurry, even though ive become pretty anal about tightening everything up on the camera before an exposure-

    Thanks

    dan

  2. #2

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    focal plane of ground glass not in sync with film

    first and simplest method is to take it to a camera repair shop who can do it for you.

    second method requires a depth micrometer and rigid metal bar of equi thickness along its length with three holes drilled through it which are big enough for the depth micrometer bar to fit through.

    The metal should be long enough to be able to rest on both sides of a film holder and the holes should be drilled with one in the centre and the others 2 inches either side of the center hole.

    now measure the thickness of the metal bar (m) with the micromter.

    next put a piece of film in one of your film holders and take 9 depth readings by resting the bar across the film holder and measuring through the holes to the film. i.e. one end, corners and center, middle of film holder, edges and center, other end corners and center. Next average the 9 readings and subtract the depth of the bar from the averaged readings. That gives you depth of film plane in a film holder.

    Next take off camera film back and make another 9 readings from surface where film holder will rest to the ground glass surface (done in same way as measuring film holder, i.e. through metal bar). Avearge of 9 readings should be same as average depth of film holder.

    If glass depth is not the same then you need to adjust when inserting new glass. How you do that depends on your film back. It may require new shims which can you work out thickness provding you have a normal micrometer to measure their thickness and calculate required thickness. On some cameras backs you have adjusting screws. Their may be other methods of adjustment. After adjustment and insertion of new glass, re measure glass depth to be sure its correct.

    N.B. if your camera uses shims then make sure you number them and their position on the back when you take them off. Otherwise you will not know which ones require replacing and where the replacements go when putting it back together again.

  3. #3

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    focal plane of ground glass not in sync with film

    Instead of the rigid metal bar with holes in, I got a glazier to cut me several strips of 4mm float window glass from the same sheet. I checked with a micrometer that its thickness was sufficiently constant, and used the strips to support my depth micrometer. I was checking my 4"x5" film holders etc., and 8" x10" could be too large for the stiffness offered by my glass strips. I did check for this on the 4x5, and thought it was adequate.

    I found it necessary to take many measurements and average the readings. Got bored fairly quickly, and so went out and took pictures at f22 rather than f5.6! (on my EOS10D of course)

  4. #4
    Robert A. Zeichner's Avatar
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    focal plane of ground glass not in sync with film

    A couple of questions:

    1. Does your Rajah have a Fresnel brightening screen? This is a piece of plastic that appears to have many hundreds of concentric circles engraved in it.

    2. If the answer is yes, it is possible that someone "modified" the camera by adding this. Putting a Fresenel between the ground glass and the lens will most assuredly cause focus problems if the camera wasn't specifically designed to have such a screen. I don't know if this is the case with the Rajah, but my guess is that it wasn't. So if you see a Fresnel, try removing it and test the camera with just a ground glass.

    3. If the answer to 1. was no, is the frosted side of the ground glass toward the lens (which it should be) or is it facing your eyes?

    4. If the answer to 4. was the latter choice, simply turn the ground glass around and that will most likely fix your problem.

  5. #5

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    focal plane of ground glass not in sync with film

    An optical method for checking the position of the ground glass vs the film:

    1. Arrange the following close-up shot. A ruler laid flat or slightly tilted (far end elevated somewhat) on a table. Use a fairly long lens wide open and close for a very shallow depth of field. The longer, wider and closer, the more accurate this method is.

    2. Focus carefully (use a magnifier. I use an 8x) on, say, the six-inch mark. The depth of field should be as shallow as possible so that only the six-inch mark is in focus. Move in closer if this is not the case.

    3 Meter and shoot wide open. Shoot a several sheets refocusing between just to eliminate errors.

    4 Develop and inspect (I usually do this test on days that I am printing and just toss the negs into the print developer for 3-4 minutes).

    Now, interpret your results:

    If the six-inch mark is in focus as you saw it, the ground glass and film plane are in the same position and all is well.

    If the area of sharp focus is in back of the six-inch mark, it means the ground glass is farther from the lens than the film. You need to remove shims or otherwise modify.

    If the area of sharp focus is in front of the six-inch mark, it means the ground glass is closer to the lens than the film. You need to add shims.

    At this point you can measure or add/remove shims by trial an error to find the correct position. The trial-and-error method requires repeating this test, sometimes two or three times.

    Do make sure that your Fresnel/ground glass is installed properly, etc. first.

    Hope this helps,

  6. #6

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    focal plane of ground glass not in sync with film

    Some more notes.

    The technique of photographing a ruler minimizes any error in the GG position because extending the bellows for close up work increases the depth of focus.

    Final test should be to photograph something at infinity such as a tree on the horizon and check it for sharpness. At infinity focus any out of sync ground glass error will be maximised.

    NB. If doing the depth micrometer test, turning the micrometer until it clicked was too much because the film and plate are flexible and don't provide sufficient resistance until compressed by the depth micrometer. Watch for the point at which the film starts to move. Having a light shine the film helps with this.

  7. #7

    focal plane of ground glass not in sync with film

    My judgement is that the job can be done with a depth micrometer without using a plate with holes. Most depth micrometers have wings that extend to sides that will work well enough. But it takes experience to use a micrometer -- a machinist would call it "feel" -- to measure to 0.001 inch accuracy you can't force the tool.

    If you decide to make a photographic determination of the error (i.e., the ruler method), the correction doesn't have to be hit or miss -- from the measured error in object distance between intended focus and actual focus, you can calculate the error in the image distance. The equation for focusing a lens is near the top of the Lens Tutorial, http://www.photo.net/learn/optics/lensTutorial. Even if you don't achieve the ideal of a perfect correction on your first try, calculating a correction in the ground glass position should converge much faster than guessing.

  8. #8

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    focal plane of ground glass not in sync with film

    Chances are your camera has a plate, rather than a film back. You might be better off finding a regular back that can be fitted to it.

  9. #9

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    focal plane of ground glass not in sync with film

    theoretically you could calculate the error from the photographic determination but making all the correct measurements would be far more difficult than using the depth micrometer method and besides, you will need to use a micrometer to make the adjustments of the film back anyway. Far simpler and much quicker to just use the micrometer method and check final position with infinity image sharpness test.

  10. #10

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    focal plane of ground glass not in sync with film

    Rob,

    If you are still reading this thread, I am curious as to your reasoning behind the statement that extending the bellows for close up work increases the depth of focus. I would think that the opposite would be true (but it wouldn't be the first time I had got things backwards), and, even if it were the case, would be offset by the shallower depth of field that occurs when the magnification increases.

    The infinity test does seem a good idea for a check, however, and one could determine which direction to reposition the ground glass when infinity is not correctly by intentionally defocusing a bit on way and the other and keeping careful track of which neg is which. I'll give it a try someday soon.

    Best,

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