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Thread: Need help with Lens to Film Distance

  1. #1

    Join Date
    Sep 2011
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    Need help with Lens to Film Distance

    Hi all,
    Just about to build a fixed focal length, hyper'-focus camera, using a 65mm 4.5 Rodenstock Grandagon-N and need some help finding the distance requirements from the lens to film.

    The data sheets state that this lens has a Flange Focal Length of 70.0mm (Based on Copal Shutte for Scale 1:Inf')

    Does this mean the distance from the front (forward face) of the lens board to the film when focused to infinity?

    I've read a few posts on Nodes etc and got very confused...so some one might have to explain in simpler terms (Sorry).

    Also, how would I work out the travel of helical required to go from infinity to say 3 or 1 meters?

    As this will be my first build, I don't want spend a fortune and will be buying a cheap 'Evilbay' Helicoil and having a play (I already own the lens from my LF gear). I would just like to know the distances first to find the best suited helicoil. Once I have the Helicoil, I will install a temporary G/Glass and mark the focus knob with set distances.

    Thanks in advanced.

    Mark

  2. #2

    Join Date
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    Re: Need help with Lens to Film Distance

    Back of shutter to the film plane. If you get the Rodenstock helical for the 65 Grandagon you would not have to worry about the travel but it would also be calibrated for depth of field. The Rodenstock data sheets from when the lens was still current would also give you the focusing range of the lens on the helical. Since it is a wide angle, not corrected for close focus, I don’t believe it would focus as close as 3 feet.

  3. #3

    Join Date
    Jan 2007
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    Re: Need help with Lens to Film Distance

    The equation you need is called the thin lens approximation. It goes like this...

    1/f = 1/u + 1/v
    where,
    f is the focal length of the lens (use 70.0 mm in your case),
    u is the object distance - the distance from the lens flange to the subject you are focusing on
    v is the image distance - the distance from the flange (close enough) to the film.

    you're building a camera so, you want to know the distance the lens must be from the film to focus on something in front of the lens...this is v, the image distance in the above equation. so, let's first re-arrange the above equation...

    (1/f) - (1/u) = 1/v --> v = 1/ ((1/f) - (1/u))


    Now, let's work through some examples....

    Let's say you want to focus on something at 1 meter = 1000mm
    v = 1/ ((1/f) - (1/u))
    v = 1/ ((1/70.0) - (1/1000))
    v = 75.26 mm

    Suppose you want to focus on something 3 meters = 3000mm away.
    Then the required flange to film distance is,
    v = 1/ ((1/f) - (1/u))
    v = 1/ (1/70) - (1/3000))
    v = 71.6723 mm


    According to this dof calculator
    for a 65mm lens on 4x5 film at f/11, the hyper-focal distance is around 3800 mm
    v = 1/ ((1/f) - (1/u))
    v = 1/ (1/70) - (1/3800))
    v = 71.3 mm
    Last edited by BradS; 10-Apr-2018 at 22:52.

  4. #4
    Nodda Duma's Avatar
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    Need help with Lens to Film Distance

    The flange focal length is NOT strictly from the back of the shutter, but referenced from the surface on the copal shutter that mounts against the lens board. So like you guessed, measure or set the distance from the front of the lens board to the film plane to be 70mm when at infinity focus.


    BradS’ post is good aside from his error of using the flange focus distance, but he doesn’t quite extrapolate the discussion to answer your question.

    - Find the values for”v” as he describes for object distance at infinity (answer is 65mm), and for your desired close-focus distance (answer is 69.52mm for 1m object).
    Close-focus distance calculated as above, infinity focus calculated by setting your “1/u” value to 0. That’s a self-check, since it equals your 65mm focal length, formally called the effective focal length.

    - Take the difference of those values to find your total range of focus travel (a few mm). This is the total distance the lens will move away from the film plane when you rack focus from infinity to your close-focus distance. 4.52mm for infinity -> 1m close focus.

    - Divide the calculated focus travel by your helicoil thread pitch. You now have total # of turns of your focusing mechanism needed to rack through your total focus range. For 1mm pitch, that’s 4.52 turns to rack focus in our example. For standard thread pitch, convert to inches (4.52mm = 0.178”). A 20 tpi thread, for example, has a 0.050” pitch. .178/.050 = 3.56 turns. Btw, these common thread pitch examples should be a good selection for your need (based on my professional experience designing optics and imaging systems), although some prefer slightly finer or coarser pitch.

    Note the amount of the error if you incorrectly use flange focal length in your calculation.

    -Jason
    Newly made large format dry plates available! Look:
    https://www.pictoriographica.com

  5. #5

    Join Date
    Sep 2011
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    Re: Need help with Lens to Film Distance

    Many thanks all for the info, I think that answer all the questions.

    The data sheet for my lens states its design for close distances (which is what I used it for) and say the lens can focus lower than 1 meter. Although I can't see me using it that close.

    The only reason I don't want to use the Rodenstock Heilcal was price, I was trying to keep to a budget. I always have my 4x5 Chamonix when I want to head out with all my gear.

    So, in theory, If I was to buy a 'user' 65mm MF lens large enough to used the helical from (That would fit my lens) I would set the infinity focus point at infinity on the G/G and the focus travel for the focus scale would be the same as the Rodenstock? (Obviously a floating element design wouldn't work).

    The Mamiya Press 65mm lens looks like I could swap the lens and shutter with mine easily (Probably not as easy as it looks!!).

    But likewise, could I install my Rodenstock in the Mamiya press and use it on the press with the rangefinder?

  6. #6

    Join Date
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    Re: Need help with Lens to Film Distance

    I believe, could be mistaken, that the 65/6.3 Mamiya and 65/4.5 Grandagon N are both in #0 shutters. If so, swapping cells requires only unscrewing and rescrewing.

    Using the Grandy on a Mamiya Press with the RF may not work as you'd like. This because the two lenses flange-focal distances are probably different. Different prescriptions ...

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