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Thread: What a difference half a millimetre makes...

  1. #11

    Join Date
    Apr 2011
    Location
    Hemel Hempstead, UK
    Posts
    462

    Re: What a difference half a millimetre makes...

    Hence my initial post here: http://www.largeformatphotography.in...lass-plane-MPP

    Doremus, a set of shims for the film holders was something I considered but there are a number of issue that persuaded me against it:
    - the shims would have to be fixed to the holders, which would mean I couldn't use them with anything else
    - the difficulty of getting the shims made flat (thickness is easy)
    - the difficulty of fixing the shims to the holders - glue thickness and lumps could be an issue

    So I have a design for a new focus holder which I am looking to get made in Delron. I'd rather make it myself (just for the fun) but I can't mill that large or that precisely on the depth; this needs a professional setup. I'll post some pictures when I get it organised (might be a while; it's a complicated time of year).

    I'll probably be dragging the camera off to Berlin over xmas, so hopefully I'll have something sorted; if not then the focussing forward approach will have to do, guesswork and all.

    Neil

  2. #12
    Jac@stafford.net's Avatar
    Join Date
    Dec 2012
    Location
    Winona, Minnesota
    Posts
    4,079

    Re: What a difference half a millimetre makes...

    As mentioned earlier, .5mm is huge.

    If focus is short of the GG, then shim the GG on its underside,
    placing shims between the GG and lens.

    Only four are necessary. Good flat shims are available. You can trim their
    length and width with a razor. I will post sources if you wish, but a good
    start is to Google "feeler gauge shims"

    I have found couple wood backs had apparently been refinished, reducing
    GG set-back - a bad thing but fixed as above.

    Moving the GG towards the lens is a matter of first checking if there are
    already shims in the place mentioned above. Remove them and replace
    with thinner shims. I've found one such case.

    If none, then machining the place where the GG holder seats (the back)
    is necessary. IMHO that is not a hand work operation. A professional
    machinist would find it trivial. I am not one.

  3. #13

    Join Date
    Apr 2011
    Location
    Hemel Hempstead, UK
    Posts
    462

    Re: What a difference half a millimetre makes...

    No existing shims, Jac; it was designed for glass plates and wooden holders. The gg casting has four 'feet' that are machined to provide the correct offset, but they're only 0.3mm above the casting. There are six contact points on the inside, on which the glass sits; that's where the machining would have to be done. Possible, but as you say, not a home job.

    Neil

  4. #14

    Re: What a difference half a millimetre makes...

    Suggest using brass shim stock cut then rolled smooth, deburred to size as needed. Shim stock is a common machine shop consumable that comes a a variety of thickness from 0.001" to about 0.032" thick:

    https://www.onlinemetals.com/merchan...id=1188&step=2


    Measure the distance from the film holder seating area to the GG on all four corners, edges and center, then shim or remove material in say 0.001" increments as needed to achieve overall uniformity of flatness across the GG to film holder seating area.


    Do keep in mind adding a Fresnel lens for GG viewing can alter where the lens focal point ends up on the GG-Fresnal to film holder seating area.



    Bernice

  5. #15

    Re: What a difference half a millimetre makes...

    Double post. deleted.

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