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Thread: Lens board opening tolerances

  1. #1

    Join Date
    Oct 2003
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    Ottawa, Canada
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    640

    Lens board opening tolerances

    I am looking at making a couple of lensboards for a Wisner and have a question on the lens plate drilling hole tolerances.

    What is the tolerance on this? For example, a Copal 0 is specified as having an opening of 34.8mm. Not surprisingly, this is not readily available. I can get a 34.925mm (1 3/8") or a 35mm bit. A small amount of run-out in the drill press is also likely to cause it to be every so slightly larger.

    Are these within acceptable tolerances? Will I see problems with a slightly oversize hole?

  2. #2

    Join Date
    Feb 2001
    Location
    Greenbank, WA
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    2,602

    Lens board opening tolerances

    Paul: Having "wood butchered" my way through a few of these projects I can comment based on experience. If you're worrrying about just 0.1 mm of oversize, you'll have no problems at all. (This is 0.05 mm of extra space on each side of the lens and inconsequential.) In truth, you can be several millimeters off and a lens will mount without problems and be light tight with a lens tightened down. Be careful to relieve enough material on the back of the board so that the retaining ring on the shutter screws far enough forward that it doesn't hit the shoulder of the lens when you screw in the back element. If this is off, it may feel like it installed just fine but the lens elements are further apart than they should be. Measuring is best, but when you screw in the rear element, you can feel when you have a problem because it doesn't stop with authority in the same place each time. I discussed this clearance issue with Mr. Grimes one time and he told me he received many lenses with complaints of poor lens performance who had the elements too far apart for this reason. A stock factory Deardorff board, for example, is so thick it will cause a problem with many, many modern lenses if you don't use a router on it. Good luck.

  3. #3

    Join Date
    Sep 2003
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    1,794

    Lens board opening tolerances

    I got a Xenar mounted on a metal board with the camera I bought. When I took the lens off the board it looked like somebody had cut the hole with a chisel. Maybe an axe. Pretty it wasn't. But I'm guessing the lens and board had been together a long time and had produced many photographs.

  4. #4

    Lens board opening tolerances



    If you study the retaining ring of the lens that you plan to mount, you can figure out the tolerances.





    The subtlety is that the diameter of the lensboard hole is not determined by the mounting threads on the shutter, but instead by a projecting lip on the retaining ring. This lip is supposed to go inside of the lensboard hole, no doubt to make a more complicated path that light can't get through. The outside diameter of this lip sets the minimum lensboard hole diameter. Another fairly common mistake is to size the lensboard hole for the shutter threads, in which case the narrow lip is incorrectly what seats against the lensboard.





    The maximum diameter of the lensboard hole should be set so that no matter how the lens is position (sideways and up/down) in the hole, the retaining ring completely covers the hole. Measuring a retaining ring for a Copal 0 shutter, the shoulder past the lip is 2 mm, i.e., the retaining ring radius is 2 mm more than the radius of the lip, which means 4 mm more in diameter. This shoulder is what is supposed to rest against the back of the lensboard. If you make the hole with a radius smaller than half of 2 mm more (i.e., 1 mm more) than the radius of the lip, then no matter how the lens slides, the ring will still completely cover the hole. For a lip diameter of 34.8 mm, this would be a hole diameter of 36.8 mm. This reasoning allows a fairly large tolerance on the hole diameter.





    Ideally the hole diameter tolerance would be held much tighter so that the retaining ring covers the hole even where it has notches for the wrench used to tighten it. Since most machining is a subtractive process, it is better to drill a hole too small and file it out then to drill it too big.





    Besides the problem that Keven has mentioned, the other reason that the lensboard shouldn't be too thick is that you want sufficient thread engagement between the shutter mounting threads and the retaining ring. About three threads of engagement is sufficient -- the retaining ring for a #0 shutter that I am looking at only has about five threads so it needs to be screwed on far enough to be using most of its threads. You can figure out the maximum allowed thickness for the lensboard by screwing the retaining ring onto the lens mounting threads by at least three full turns and then measuring the space between the shoulder of the retaining ring and the back of the shutter. Lensboard thickness tends only to be an issue for wooden boards. Schneider has some tables for various shutters that include values for the allowed range of lensboard thicknesses: http://www.schneideroptics.com/photo...ries/shutters/. For Copal 0 Schneider says 1.5 to 4.0 mm, while for Copal 1 they say 1.5 to 3.0 mm thick.


  5. #5

    Join Date
    Oct 2003
    Location
    Ottawa, Canada
    Posts
    640

    Lens board opening tolerances

    Wow, thanks for all the help! I feel much better (not to mention more informed) now. So I will drill the hole as close as the bits allow and route out a rabbet to get the thickness within acceptable limits.

    Thanks very much for the help; it will really help in the construction.

  6. #6

    Lens board opening tolerances

    Paul;

    I have just made 2 lensboards for a Deardorff 4x5 for a total of $10.

    I took a stock Polypropylene CD case with 1 mm thick shell walls, and a standard DVD case with 0.5 mm walls, cut out squres 4 in. to a side, then a third 0.5 mm thick piece of plastic 3.75in to a side. The smaller piece is centred to locate the board inside the lower of the two levels in the 'Dorff front face as well as create another light trap.

    I then put centre pilot holes in all 3 pieces and cut the appropriate diameter circle with a heavy box cutter, by hand around a penciled circle. I polished up the inevitable irregularities in the hand-cut circle with a small file.

    I added 4 thin metal strips to each board, one to each side clearing the hole for the lens. Then I centred the 3 sheets and put two 3/8 in nuts and bolts thru on each side and thru the metal strips and painted them all flat black.

    The lens hole was +/- 1-2 mm but this was taken up by the lens backing ring AND some black duct tape applied radially at teh inner circumference of the opening. This acts as gasketing material and seals it nice and snug.

    2 Solid boards for lesss than $10 and 1 hr work!!!

    Cheers

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