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Thread: Ground glass film alignment?

  1. #1

    Ground glass film alignment?

    After having some eradict focussing issues, I decided to investigate the issue o f ground glass / film alignment. I read the last thread, however it did not add ress some of the issues I thought of. For consistent critical focus I realize the film must sit in the exact sam e plane as the ground glass, i.e. so the film sees exactly what I see in the gro und glass. However, when studying how to test this, I came up with a good syste m, but many questions. Using a depth micrometer with a large straight milled me tal bar, I can measure from the rear standard plane down to the glass. Comparin g that distance to the film holder in place with film would make sense. However this raised several questions which I could not find answers to.

    1) Since the ground glass and film are different thicknesses, where do you meas ure the micrometer to for proper comparison? How about fresnel lenses, how do t hey affect the measurement?

    2) Is the actual focus point on the ground glass on the back where your eye is, or in the front towards the lens?

    3) Once it is determined what you should be measuring to, then what would be a reasonable tolerance between film holders, quick loads, polaroids, roll film bac ks, etc? The micrometer for $30 reads to .001 inch.

    This seems like such a critical issue, I find it hard to beleive this is n ot tested more often and a micrometer kit is not sold for 4x5 and 8x10's with in structions. Anybody have experience at this? It will save me a lot of headache s, thanks in advance...

  2. #2

    Ground glass film alignment?

    When I bought a used Horseman I shot a roll to test the lenses. Much to my surprise the shots taken wide open (of a newspaper page) were horrible. I wondered if the ground glass/film plane were the same- they were not. the previous owner had the ground glass in reversed! A depth gauge in thousanths revealed the major discrepancy. I don't think film thickness has too much to so with it .003"-.004".

  3. #3

    Join Date
    Mar 1998
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    1,973

    Ground glass film alignment?

    What means "eradict"?

    1.) Ideally, the film plane and the front of the groundglass and or groundglass intensifier combination are the same. I say ideally because film in virtually al l holders, except the very exotic and expensive Sinar and vacuum platen holders, curves very slightly, especially if you are shooting at a severe downward angle with large (8x10 and up) size film.

    2.) On the lens side, where ground grain of the glass is.

    This is one of those questions that proves that theorectically at least, photogr aphy is impossible. If you are having sharpness problems and are using good equi pment more than likely the problem lies elsewhere. Possibly in your technique, p ossibly in the way your camera or lens are put together, but more than likely th e problem is not with groundglass to film plane alignment. Recently Bill you pos ted about prontor shutters. If you switched to prontor shutters did you do the s witching or did you have a qualified repair man like Steve grimes do it. Have yo u taken your lenses apart to clean them? Have you had your lens tested on a collimeter? How powerful a loupe are you usin g? You don't need to be using more than a 4x. You are shooting landscapes correc t? Could wind be vibrating your tripod? There is literally a near infinity of qu estions to be answered. If you bought your lenses from an overseas source and th ey are dogs, are is the dealer going to make them right? etc., etc.

  4. #4

    Join Date
    Mar 1998
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    1,973

    Ground glass film alignment?

    One last thing. The reason micrometer kits are not sold with LF cameras is becau se they aren't generally (99.9% of the time) needed, especially with high end ca meras where precision is what you are paying for. if you are having this headach e with both your new 4x5 and 8x10 cameras and their respective lenses then I thi nk that the error must lie with the operator.

  5. #5

    Join Date
    Jun 1998
    Posts
    53

    Ground glass film alignment?

    Just to prove how right Ellis's statement is, after all the hours I spent once i n calibrating, checking, rechecking, emailing, etc., it turned out the previous owner had simply installed the screen wrong. A quick call to the screen's mfg would have saved me hours. It was educational, however.

    There's a simple test. Place a yardstick on the wall and photograph it, focusing on, say, 18". Do this reasonably close and from a 45 degree angle. If your lens is sharp wide open, use that aperture or c lose to it. Focus with a good loupe, develop the film and use a high power loupe to confirm 18" is the sharpest point or within the sharpest area.

  6. #6

    Ground glass film alignment?

    Thank you for the responses... a few comments on the above, the camera equipment is all new Toyo, VX125 with the same film holders. The lenses are all new Nikkor and Fuji purchased from B&H and Badger. There are no prontor lenses installed. The ground glass / fressnel is installed properly. As for eradict focussing issues, I was refering to shots that were taken many times with the same camera position, yet some were not as sharp, there was no wind at all and I never changed the f stop or the focus position, I only altered some filters in my Lee holder. This led me to beleive there was film alignment problem in some of the film holders? I agree with a lot of the issues you all raised, maybe it makes sense to shoot some test targets with the same film holders I used on the above trip, this would isolate any film holder problems without trying to measure them with a depth micrometer? Does this make sense as a "peace of mind check?" I also will try the yardstick test, it seems to confirm the lens though, correct?

  7. #7

    Join Date
    Jun 1998
    Posts
    53

    Ground glass film alignment?

    It's, of course, not totally impossible for an mfg to have something wrong or a lens to be out of whack (or is that "in whack" if it's wrong?) However, I would certainly suspect something other than y our equipment based on the VX and the new lenses.

    I've made a pact with myself: I bought all new film holders and I intend to do s o on a periodic basis, replacing the last set. When they are new or like new they are very smooth, no warpage and tight. S ure makes life easier.

    The focus test on the yardstick will test your system, not just one component.

    HTH.

    M

  8. #8

    Ground glass film alignment?

    A few years ago, I took a workshop taught by Joe Englander. At the first session, using a micrometer system, he measured the groundglass placement of all the camera being used by everybody in the workshop. Amazingly, almost every one was off. The moral of the story seems to be that just because we pay lots of money in the name of precision doesn't necessarily mean that we get it.

  9. #9

    Ground glass film alignment?

    Thank you Rob for that response. Now what were instructed to do about this? What did you measure the micrometer to? What were the tolerances acceptable? Did you fix your camera?

  10. #10

    Join Date
    Jan 1998
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    262

    Ground glass film alignment?

    Maybe it is just because I use crummy equipment manufactured well before the war (that would be WWI, not the war between the states) but I can't imagine focusing to the tolerances these two threads are discussing. Can anyone really focus to the 1/100th of an inch? But people here are talking about 1/1000ths . . ..

    Erik

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