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Thread: Where is the image formed on the ground glass? (With and without fresnel) | image reg

  1. #1

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    Where is the image formed on the ground glass? (With and without fresnel) | image reg

    I am trying to identify exactly where my film sheet should be positioned relative to the image formed on the ground glass. Further I would like to know the various effects of ground surface size/roughness and the effects of fresnel addition.

    My understanding is that only one surface of the glass is ground. The actual ground surface forms an image by catching/“refracting” the rays passing through the glass. Am I correct in then assuming that this is the image forming plane that needs to be aligned with the film emulation?

    Does anything change with a fresnel lens placed in front of the ground glass?

    Which part of the film should the image plane be aligned to? The middle of the emulation layer? Does anyone have references on the various film layer thicknesses?

    Is there a technical reference I can read more about the ground glass, image formation, fresnel effects and design, and film registration?

  2. #2

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    Re: Where is the image formed on the ground glass? (With and without fresnel) | image

    The image is always formed in the frosted side of the GG, it is a real image (not a virtual one).

    If a fresnel is placed in the outside then the inner frosted side has to match the film position. If the fresnel is in the inside then a distance correction may be required to keep focus.

    If you want to keep it simple then place the fresnel in the outside.

  3. #3

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    Re: Where is the image formed on the ground glass? (With and without fresnel) | image

    Quote Originally Posted by l2oBiN View Post
    ... Is there a technical reference I can read more about the ground glass, image formation, fresnel effects and design, and film registration?
    Try searching here and over on Photrio.com. There's a lot of information to be gleaned, including ISO specifications for filmholders, etc., etc.

    As Pere points out, the image is formed on the frosted side of the ground glass (where you correctly assumed that the light is scattered). Cameras and filmholders are designed to place the surface of the film in the same position as the image forming surface of the ground glass unless there is a Fresnel lens placed between ground glass and lens. In the latter case, there is a focus shift of roughly 1/3 the thickness of the Fresnel lens sheet. Several LF camera models are designed with Fresnel lenses between image-forming ground glass and lens. The makers ensure that the spacing of the ground glass is correct so that the image is formed correctly on the film (i.e., the ground glass is not in the same position as on cameras with just a plain ground glass). One needs to take care when replacing ground-glass/Fresnel sandwiches on these cameras that one gets the correct orientation of screen/Fresnel and that the thicknesses are the same as the design specs or else focus can be affected.

    Your inquiry begs the question: What are you trying to do? Design a camera? Install a Fresnel lens? Correct a focus problem? Or just learn? Or something else? You'll get more specific help if your question is more specific.

    Best,

    Doremus

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    Re: Where is the image formed on the ground glass? (With and without fresnel) | image

    Let me add:

    -> Shet film thickness is 0.007" to 0.009".

    -> A 4x5 film holder is ANSI specified as having a +/-0.007" tolerance for the film position. For 4x5 the septum depth from the holder flange is 0.197" with that +/-0.007" tolerance, so the film septum if positioned with a 0.014" precision.

    -> The emulsion thickness is a very small fraction of the entire film thickness, most of the thickness is the plastic base, so basicly you want to focus on the film surface.


    Think that a shot has DOF, so precision beyond certain level is not required as it provides no practical benefit, other factors limit optic performance more than a 0.1mm "error" in focusing. In fact you cannot focus exactly simply because grains in the GG do allow to see all resolving power. No problem, we usually focus wide open and later we stop to increase DOF, this is usually good as scenes are 3D and almost nothing is in perfect focus, but in the DOF range.


    See page 146 https://books.google.es/books?id=tYm...20ansi&f=false

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    Re: Where is the image formed on the ground glass? (With and without fresnel) | image

    Quote Originally Posted by Pere Casals View Post
    Let me add:

    -> Shet film thickness is 0.007" to 0.009".

    -> A 4x5 film holder is ANSI specified as having a +/-0.007" tolerance for the film position. For 4x5 the septum depth from the holder flange is 0.197" with that +/-0.007" tolerance, so the film septum if positioned with a 0.014" precision.

    -> The emulsion thickness is a very small fraction of the entire film thickness, most of the thickness is the plastic base, so basicly you want to focus on the film surface.


    Think that a shot has DOF, so precision beyond certain level is not required as it provides no practical benefit, other factors limit optic performance more than a 0.1mm "error" in focusing. In fact you cannot focus exactly simply because grains in the GG do allow to see all resolving power. No problem, we usually focus wide open and later we stop to increase DOF, this is usually good as scenes are 3D and almost nothing is in perfect focus, but in the DOF range.


    See page 146 https://books.google.es/books?id=tYm...20ansi&f=false
    You really don’t usually stop down just to increase DOF. You should be stopping down to the optimal performance of the lens, usually f22, unless you deliberately are looking for something other then optimal performance.
    And, if you are looking for maximizing DOF, at any aperture, then you have to learn where to focus for the maximum DOF.

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    Re: Where is the image formed on the ground glass? (With and without fresnel) | image

    Quote Originally Posted by Bob Salomon View Post
    You really don’t usually stop down just to increase DOF. You should be stopping down to the optimal performance of the lens, usually f22, unless you deliberately are looking for something other then optimal performance.
    And, if you are looking for maximizing DOF, at any aperture, then you have to learn where to focus for the maximum DOF.
    Bob, of course... we set aperture for aesthetics, for maximum performance or for DOF...

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    Re: Where is the image formed on the ground glass? (With and without fresnel) | image

    Or for all three if we are lucky!

    Before setting up the camera, I will often take a quick meter reading to see what is possible in shutter speeds and f stops combinations for the light and wind that is present and the film I am using. Useful info when crafting the image of the GG.
    "Landscapes exist in the material world yet soar in the realms of the spirit..." Tsung Ping, 5th Century China

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