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Thread: Fresnel lenses and ground glass focusing screens your worries answered

  1. #41

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    Re: Fresnel lenses and ground glass focusing screens your worries answered

    Sweep,

    Your Fresnel sandwich has to include a frosted surface for the image to be focused on. In essence, it is a ground glass and Fresnel lens in one. Wista screens are made this way as well. So if it is deformed, the frosted image-forming surface is as well and your focus will be slightly off. Do try to get it back to flat.

    Best,

    Doremus

  2. #42

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    Re: Fresnel lenses and ground glass focusing screens your worries answered

    Coming back to the discussion!

    From Sweep, Yorkshire:
    Absolutely right. It may have deformed over time

    Well, sorry, I did not understand that your Fresnel lens was also the ground glass! And that your PerspexTM (** note 1) sheet was just a protective overlay.

    Exactly like in the post-1958 Rolleiflex TLR's focusing device, which is a molded plastic Fresnel lens with a very fine pitch and a frosted "look".

    Hence, as usual , Doremus is right, it's better if your "ground glass" is perfectly flat!

    However, the situation is not really catastrophic, taking into account a reasonable estimation of depth-of-focus in the 10"x8" (** note 2) format.
    For far-distant objects, the focusing tolerance around the sharp image plane is plus or minus N c, where "N" is the f-number and "c" is the diameter of the circle of confusion chosen as a sharpness criterion.
    If we take c = 0,17 mm (0,17 mm = 300 mm /1720, 1/1720 = 2 minutes of arc) and N=22, we get a depth of focus of plus or minus 22x0,17 = plus or minus 3,7 mm. Hence you can safely tolerate about 2-3 mm of bulging for your Fresnel+GG combo.



    (** note 1) The first time I read the word "PerspexTM " was in a book entitled Spitfire at war by Alfred Price.
    I had never heard about "PerspexTM " before, because Poly-Methyl-Metacrylate (PMMA) plastic changes name when crossing the borders!
    In France we sometimes call it AltuglasTM but more often "Plexi" for PlexiglasTM.
    For me, "PerspexTM " is like the RADAR, legendary British technology of WW-II!
    In Spitfire at war, the author explains how the ground staff had to carefully clean and polish "PerspexTM " canopys, it was a matter of life and death for RAF pilots, stray light being the enemy, according to the legendary rule established during WW-I: "The Hun is always in the Sun".

    (** note 2) 10"x8" is supposed to be the proper British denomination for a film format that we call 20x25 cm in France

  3. #43

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    Re: Fresnel lenses and ground glass focusing screens your worries answered

    Lucite™

  4. #44

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    Re: Fresnel lenses and ground glass focusing screens your worries answered

    Thanks Doremus for your guidance and thanks to Emmanuel for your entertaining reply
    I have only shot eight sheets of 10x8 so far and all appear fine on the focus, although potentially having focus appear to change radially from the centre, apparent at fully open diaphragm, doesn't help my learning curve much!
    Whilst you say tom"ay"to and I say tom"ar"to, and you say 8x10 and I say 10x8, it appears that ICI chemists in England first discovered PMMA in the 1930's and bestowed upon it the name Perspex so, being English, I will cling on to this small comfort and recall the days when we actually developed and made things that were useful.

  5. #45

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    Re: Fresnel lenses and ground glass focusing screens your worries answered

    Some cameras are designed so that you can place the fresnel in between the lens and the ground glass with a proper slot to sit the fresnel. I remember reading that if your ground glass is fine then it benefits from having the fresnel in front of it, between the lens and the ground glass. I remember doing that with a Hopf ground glass and there was a noticeable difference. My camera has a provision for the fresnel to be put in between the lens and ground glass.

  6. #46
    Jim Jones's Avatar
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    Re: Fresnel lenses and ground glass focusing screens your worries answered

    Quote Originally Posted by Daniel.E View Post
    Some cameras are designed so that you can place the fresnel in between the lens and the ground glass with a proper slot to sit the fresnel. I remember reading that if your ground glass is fine then it benefits from having the fresnel in front of it, between the lens and the ground glass. I remember doing that with a Hopf ground glass and there was a noticeable difference. My camera has a provision for the fresnel to be put in between the lens and ground glass.
    Remember that placing the Fresnel between the ground glass and lens will change the optical path as Doremus so thoroughly described in post #29.

  7. #47

    Re: Fresnel lenses and ground glass focusing screens your worries answered

    Years ago, I bought a Chamonix 45N-1... the one with the focus problem. I bought a Maxwell screen and had it installed by the technician Maxwell always recommended. It's a custom fit to the Maxwell screen.

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