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Thread: Smarter material for ground glass shim?

  1. #21

    Join Date
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    Re: Smarter material for ground glass shim?

    It's not just the thickness of old fresnel, but due to new optical distance added by fresnel...

    Check graflex.org for the proper distance there, I seem to remember it around .047" (?) about thickness of a popsicle stick that can be sanded to the thickness, or brass strip (available in USA retail) was close... Four little pieces on 4 sides... Then do a close-up focusing test wide open of three items like playing cards in a row focusing on the middle card, and checking focus on developed film...

    I did this, and worked well...

    Steve K

  2. #22

    Join Date
    Nov 2017
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    290

    Re: Smarter material for ground glass shim?

    Quote Originally Posted by LabRat View Post
    It's not just the thickness of old fresnel, but due to new optical distance added by fresnel...

    Check graflex.org for the proper distance there, I seem to remember it around .047" (?) about thickness of a popsicle stick that can be sanded to the thickness, or brass strip (available in USA retail) was close... Four little pieces on 4 sides... Then do a close-up focusing test wide open of three items like playing cards in a row focusing on the middle card, and checking focus on developed film...

    I did this, and worked well...

    Steve K
    the lens part of the fresnel is in direct contact with the gg and shouldn't cause any significant shift in the focal plane, which just leaves the potential shift caused by 1/16" of acrylic, which shouldn't be much different from 1/16" of glass if the gg is mounted backwards in the position of the fresnel. the frames with and without fresnels differ in depth by 1/16", so apparently nobody at graflex thought there was a noticeable difference in physical and optical distances.

  3. #23

    Join Date
    Sep 2010
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    738

    Re: Smarter material for ground glass shim?

    I don't like Fresnels either, especially with wide lenses. I found a type of gray plastic material in a large hobby store used for model railroads that is the exact thickness required to shim a Graphic ground glass when the Fresnel is removed. The thickness of the shim required is less than the thickness of the Fresnel lens. I had already determined what was required by measuring the film plane of a holder in place and finding that plastic in the hobby store with a micrometer was a lucky coincidence. It is used for something to do with model railroad tracks. I don't have a piece handy or I would measure it for you. It's about as thick as a bank or credit card card and about an inch wide. You need to cut strips about 3/32" wide to fit under the ground glass.

  4. #24
    BIATHLONEIL's Avatar
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    Dec 2010
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    Lunenburg
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    8

    Re: Smarter material for ground glass shim?

    Beer cans and a micrometer. There is a great variation in wall thickness. Do your research carefully but enthusiastically.
    "A closed book teaches no one".

    -Neil the Wheel

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