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Thread: Slight focus error

  1. #1

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    Slight focus error

    Critically focussed on the ground glass almost wide open aperture but focus in the print is slightly in front of the point of initial focus [nearer to camera] Do I shim between the ground glass frame and the ground glass which moves the GG closer to the object? Thanks

  2. #2
    8x10, 5x7, 4x5, et al Leigh's Avatar
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    Re: Slight focus error

    If the actual focus is closer than shown on the GG, it means the film is farther from the lens than the GG.

    Negative shim. or remove an existing shim.

    - Leigh
    If you believe you can, or you believe you can't... you're right.

  3. #3
    pasiasty's Avatar
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    Re: Slight focus error

    BTW: some small format lenses are prone to focus shift - i.e. changing slightly focus plane while stopping down. From obvious reason this can be detected only in case of very fast lenses, that gives shallow DOF even, when stopped down one or two stops. I don't know if this may also cause focus errors with large format. Anyway, unequal registration of a ground glass and a film seems to be more probable cause.
    || Cezary Żemis <cezary.zemis@pronet.pl> | www.cezaryzemis.name
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  4. #4

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    Re: Slight focus error

    Thanks Leigh, sadly no existing shim. Ah well a wood work project down the road by the looks of things.

  5. #5

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    Re: Slight focus error

    Quote Originally Posted by Shen45 View Post
    Critically focussed on the ground glass almost wide open aperture but focus in the print is slightly in front of the point of initial focus [nearer to camera] Do I shim between the ground glass frame and the ground glass which moves the GG closer to the object? Thanks
    Shen, before you start drilling and blasting, check you filmholders - or roll back if you used one.
    One man's Mede is another man's Persian.

  6. #6

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    Re: Slight focus error

    Quote Originally Posted by pasiasty View Post
    BTW: some small format lenses are prone to focus shift - i.e. changing slightly focus plane while stopping down. From obvious reason this can be detected only in case of very fast lenses, that gives shallow DOF even, when stopped down one or two stops. I don't know if this may also cause focus errors with large format. Anyway, unequal registration of a ground glass and a film seems to be more probable cause.
    Ever heard of a Goerz Dagor? They aren't fast, they aren't small format, and they're famous for growing longer as you stop them down.
    One man's Mede is another man's Persian.

  7. #7

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    Re: Slight focus error

    Quote Originally Posted by Shen45 View Post
    Thanks Leigh, sadly no existing shim. Ah well a wood work project down the road by the looks of things.
    After you've measured* the depth to make sure it's incorrect, I'd return the back to wherever you bought it. Have them fix it.

    *in the book-Way Beyond Monochrome

  8. #8

    Re: Slight focus error

    Shen, if the film is farther than the GG from the lens, you need to shim the ground glass out not in.

  9. #9
    Unwitting Thread Killer Ari's Avatar
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    Re: Slight focus error

    If you aren't a machinist, have a machinist do the work.
    It may be spendy, but it will be done properly with a minimum of frustration.

  10. #10
    Jac@stafford.net's Avatar
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    Re: Slight focus error

    Quote Originally Posted by Chauncey Walden View Post
    Shen, if the film is farther than the GG from the lens, you need to shim the ground glass out not in.
    I don't think so.

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