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Thread: Focus point

  1. #1

    Join Date
    Sep 2013
    Posts
    1

    Focus point

    Hello

    I am a student Photography at a Belgian art academy (7th year) and quite new to this community. For my last 3 years of specialisation I am busy making my own 4x5 field camera in Mahogany wood to make portraits. So far everything fits well. I must admid I spent since August more hours than first expected to build this camera (total now 80 hours). The camera is almost ready. I am now building a wooden frame that will fit on the back of the camera with the Fresnel and ground glass. I also have a plastic Ilford 4x5 film holder. I know there are some standard distances from the top of the holder to the mid (film) support and now comes my question : I need to respect the same distance so that the focuspoint on my groundglass matches perfectly (tenth's of a mm tolerance ?) with the plastic holder which will put once the focus is correct between back and groundglass holder. As my Fresnel + groundglas are +/- 2.7 mm thick I would like to know the exact focus point I need to respect :

    is it :

    - entrance at the Fresnellens at same side as the lens in front
    - the transistion between Fresnel and groundglass
    - the groundglass at my eye side ?


    If the question is not clear - please ask me I will make if necessary a drawing to explain.

    Every help is welcome.

  2. #2

    Join Date
    Dec 2001
    Location
    NJ
    Posts
    6,499

    Re: Focus point

    Most makers put the fresnel between the ground glass and the photographer, not between the ground glass and the lens. Doing this eliminates the need to calculate or measure how much the fresnel shifts the focal plane.

    The ground side of the GG faces the lens, not the photographer. This eliminates the need to calculate or measure how much the GG shifts the focal plane.

  3. #3

    Join Date
    Feb 2006
    Posts
    3,116

    Re: Focus point

    ^^ What Dan said ^^
    But, here's what I'd do: I'd build the camera without a fresnel. With the possible exception of small aperture wide angle lenses, you don't need one as long as you use an effective darkcloth. I find it easier to focus without a fresnel, on 4x5 as well as 8x10.
    One man's Mede is another man's Persian.

  4. #4

  5. #5
    Steve Smith's Avatar
    Join Date
    Jan 2011
    Location
    Isle of Wight, near England
    Posts
    702

    Re: Focus point

    The frame which holds the ground glass is made so that the ground surface of the glass facing the lens is 0.197" or 5.004mm in from the edge of the frame. That way, when it is applied to the camera back, the ground surface is in the same position as the film would be when a film holder is applied to the back.

    I have some CAD drawings of a 5x4 back you could have if they are of any use to you. Send a PM.


    Steve.

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