Switch to English Language Passer en langue française Omschakelen naar Nederlandse Taal Wechseln Sie zu deutschen Sprache Passa alla lingua italiana
Members: 70,283   Posts: 1,535,001   Online: 1039
      
Page 1 of 2 12 LastLast
Results 1 to 10 of 15
  1. #1

    Join Date
    Sep 2002
    Location
    Lehi, UT
    Shooter
    4x5 Format
    Posts
    21

    ground glass thickness

    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. #2

    Join Date
    Dec 2004
    Shooter
    Multi Format
    Posts
    2,146
    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. #3
    Dave Parker's Avatar
    Join Date
    Aug 2004
    Shooter
    Multi Format
    Posts
    4,049
    Quote Originally Posted by argus
    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
    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. #4
    SchwinnParamount's Avatar
    Join Date
    Nov 2004
    Location
    Tacoma, WA
    Shooter
    4x5 Format
    Posts
    1,012
    Images
    42
    Quote Originally Posted by Satinsnow
    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
    ... 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. #5
    Dave Parker's Avatar
    Join Date
    Aug 2004
    Shooter
    Multi Format
    Posts
    4,049
    Quote Originally Posted by SchwinnParamount
    ... 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.
    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. #6
    Donald Qualls's Avatar
    Join Date
    Jan 2005
    Location
    North Carolina, USA (transplanted from Seattle)
    Shooter
    Multi Format
    Posts
    1,845
    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.
    Photography has always fascinated me -- as a child, simply for the magic of capturing an image onto glossy paper with a little box, but as an adult because of the unique juxtaposition of science and art -- the physics of optics, the mechanics of the camera, the chemistry of film and developer, alongside the art in seeing, composing, exposing, processing and printing.

  7. #7
    Dave Parker's Avatar
    Join Date
    Aug 2004
    Shooter
    Multi Format
    Posts
    4,049
    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. #8
    Murray@uptowngallery's Avatar
    Join Date
    Sep 2003
    Location
    Holland, MI
    Shooter
    Pinhole
    Posts
    1,028
    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
    Murray

  9. #9
    Dave Parker's Avatar
    Join Date
    Aug 2004
    Shooter
    Multi Format
    Posts
    4,049
    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. #10
    Calamity Jane's Avatar
    Join Date
    Jan 2005
    Location
    Manitoba Canada
    Shooter
    Large Format
    Posts
    159
    Images
    1
    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 . . .

Page 1 of 2 12 LastLast


 

APUG PARTNERS EQUALLY FUNDING OUR COMMUNITY:



Contact Us  |  Support Us!  |  Advertise  |  Site Terms  |  Archive  —   Search  |  Mobile Device Access  |  RSS  |  Facebook  |  Linkedin