ground glass thickness

Discussion in 'Camera Building, Repairs & Modification' started by Curtis Nelson, Jul 13, 2005.

  1. Curtis Nelson

    Curtis Nelson Member

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    Hi,

    Does it matter how thick the ground glass focussing screen is? I've got some glass that's maybe 1/8" thick (from an old picture frame). Would that be thick enough?

    Curtis
     
  2. argus

    argus Member

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    I don't think that will be a problem as the grinded face of the glass must point towards the lens.
    The only critical measurement - as you might already know - is that the grinded face is in exactly the same position as the film.

    My ground glass is even a little bit thinner, only about 2mm.
    1/8" = 3,175mm

    G
     
  3. Dave Parker

    Dave Parker Inactive

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    Normally ground glass thickness is not criticle, unless you have a mount that requires the screen be slid into a slot, as long as the ground side of the screen is in the correct registration for the film plane, you should have no problems at all, the most important thing is to make sure the ground side of the glass is in the proper orientation of where the film in located in the holder.

    Dave
     
  4. SchwinnParamount

    SchwinnParamount Member

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    ... or as I learned, if your camera has a fresnel lens in front of the existing ground glass then you will still have to use that fresnel lens with a new ground glass or you won't be able to get a sharp image on your negative.
     
  5. Dave Parker

    Dave Parker Inactive

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    If you have a fresnel and decide not to use it, you will have to measure the fresnel thickness and then shim the ground glass that same thickness to maintain the correct focus registration.

    Dave
     
  6. Donald Qualls

    Donald Qualls Member

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    Well, actually, shim the glass a suitable *fraction* of the fresnel thickness. IIRC, for most acrylic screens, that fraction is around 2/3, but it's dependent only on the refractive index of the material.
     
  7. Dave Parker

    Dave Parker Inactive

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    Actually Donald, it would depend on the magnification factor of the particular fresnel you are using, as with regular lenses fresnel screens are also made to a specific magnification factor as well, or focal length.

    Dave
     
  8. Murray@uptowngallery

    Murray@uptowngallery Member

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    I seem to recall for a plate of glass (starting with the simpler case; I'm not missing Dave's point), the focal point is shifted back by t/n when a plate of glass is inserted between a lens and the film, t being the thickness and n being index of refraction.

    I don't have a number for acrylic but I recall optical (eyeglass) polycarbonate having an 'index' (I'll swagger as I shorten 'I of R' to just 'index', and act like I'm
    in optics as a profession) right in the mid-range that some optical glass does. 1.5-1.6 roughly.

    My Crown Graphic (Ektalite?) Fresnel looks pretty thick, maybe 1/10 inch. Ignoring the lens issue for a moment, 0.1/1.5 is 66.7 thousandths, enough to really botch your focus.

    For a cheapo sheet magnifier, ball park thickness 0.015", focus shift = 0.01". I forget the tolerance on 4x5 ...+/- 0.007"? but 8x10 and 11x14 might be +/- 0.010-0.015"

    I have used cheapo sheet magnifiers & hoped for the best with brightening the image, despite unknown magnification.

    So Dave, does the Fresnel factor alter the index-dependent focal point shift, or are you referring to an optimally selected one with magnification or f.l matched to the lens?

    Thanks
     
  9. Dave Parker

    Dave Parker Inactive

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    In an ideal world Murray, you would have a fresnel properly matched to a particular focal length of lens, although in the real world, I have not seen this type of precision in ground glass set ups..normally, I recommend measuring the film registration when replacing anything in the optical path, that is taking the measurement of the known focal distance that produces the correct focus. and then set the new fresenl or glass at the same and taking a shot to verify the correct focus plane. Now I know that really this is a bit more precise that most people actually do, When setting up screens. In fact, I am not even sure that most of the companies do it either, in the real world, I don't know that 66.7/1000 makes that much difference, when your talking about depth of field and stop down factors when shooting with LF lenses..if my caluculations are correct, we are talking about 0.017018 millimeter, but I might be wrong, I really have not done the calculations alot as of late...I do know that we have only had one focus problem and this was on an older Rollei Medium Format camera..and have not had any reported problems from the LF guys.

    I will have to do some calculations base on the RI of our screens, then I would probably be better equiped to give a better answer. One thing to remember is that different glass fourmulations, have different light transmission properties...based on the mineral content of the substrate..but then again, in the perfect world, you would have a fresnel that was optically matched to the lens and ground glass you intend to shoot it with and change everytime you change lenses to have a completely matched optical setup.

    Dave
     
  10. Calamity Jane

    Calamity Jane Member

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    Most of the old cameras I have worked on have the GG inserted into the frame from the film side, in which case the glass must be positioned so that the inner (frosted) surface of the glass is the same distance from the camera back as the film will be. In 4x5s, that's 0.195" or 5 mm (if I remember correctly).

    When I build my second 4x5 camera, I made my GG frame differently, so that the glass is inserted from the BACK of the camera. That way the GG frame provides the spacing and the thickness of the glass is irrelevent. Don't know why the camera makers didn't do it the same way . . .
     
  11. Whiteymorange

    Whiteymorange Subscriber

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    Changed glass for film?

    A question from an ignorant man: many of the old LF cameras that I have inherited seem to have the frosted side of the GG facing out - on the back side of the camera, not facing the lens. They are old and have been kicked around some, so I can't say that that is the way that they were made, but they seem to work pretty well with modern film holders. Is it possible that the previous owners reversed the glass to make up for the difference between glass plates and film in terms of the placement of the emulsion in relation to the lens?

    Thanks,
    Whitey
     
  12. smieglitz

    smieglitz Subscriber

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    Here's a page that lists the critical "t" distance you refer to as well as other dimensions for 4x5 and all common sheet film holders:

    http://home.earthlink.net/~eahoo/filmhold.html
     
  13. Murray@uptowngallery

    Murray@uptowngallery Member

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    Is that Whitey Morange or Whitey M. Orange?...just curious so I don't mispronounce it in my head every tiem I see it.

    My first thought was the gg was reversed by someone, and you have an interesting reason why. Hopefully someone knows how plate vs film applies to your proposed explanation.

    I just borrowed a Zone VI camera from a guy at work to study and noted he had put his GG in backwards from the first time I looked at it.

    Other than in my CG that has an Ektalite Fresnel, I do the ground side toward lens/Fresnel behind GG method for my experimental stuff (which I have only looked thru but never shot film with yet...I'm easily amused by the ground glass image for now).

    I am spatially challenged. What confuses me is whether to take film thickness into consideration or not when building a camera back, and if so, whether to make the spacing deeper so the surface of the film is at 0.197" for 4x5. I realize film thickness is within the tolerance, filmholders might be warped, etc, so in the interest of being neurotic, and having to go to the effort of spacing anyway, why not go for dead center ? Maybe paper negs are thicker than 0.007" too.

    Thanks
     
  14. Whiteymorange

    Whiteymorange Subscriber

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    It's Morange, though I often tell people to spell it the other way so that they get it first time, you know, orange, with an M in front of it. Funny thing is that the reaction I get when I say this varies by region (my wife does the same spelling thing and gets the same reaction.) In eastern MA, most people seem to get it. In other locations where we have spent some time we more often get "huh?" Must be an NE thing.

    The cameras that I have with the glass frosted in the back seem to work great. Being of the sometimes hamfisted variety of man, I'm going to leave them that way. If it ain't broke...etc.

    And Murray, what about that negative feedback??
     
  15. Murray@uptowngallery

    Murray@uptowngallery Member

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    The neg FB...I must wear it like a Badge of Courage, milk it for all it's worth, get my mileage out of it, etc.

    I am cooling off a little....so I hesitate to broadcast the name of the seller in bold letters, as they have initiated a Square Trade resolution. I got the (-), my first ever I might add, as retaliation for GIVING neg to a seller who sent me a worn, beat product with a 1988 date code, described as new then offered to exchange it. Another winning bidder on same product got one in similar condition dated 1983, and was equally unhappy.

    I put a link to pics on the web in my feedback since it appeared to be blatant fraud, not an oversight (two people in a row, come on!). Offer to exchange it I didn't respond to because they rationalized it in many ways.

    I will wait until the Square Trade thing pans out & see if the resolution was an honest fix or ...something else. If it's corrected properly, I'll undo my smear campaign. IF invoking Square Trade ends up being part of a darker plot by the seller, no reason to assume so yet, I will be out with a virtual flame thrower, to protect innocent unsuspecting fellow photographers from such indignity. I think it's too much work for most people who get burned, but I may have given them a wake-up call (only a one-alarm so far).

    I will take my chances with the average eBay clown who 'doesn't know anything about cameras' and not be surprised if an item doesn't meet description or has functional issues, but a dealer with a condition rating system posted on their website?...uh, uh, no way.