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Thread: How much bellow draw needed for 30" lens for 14xx17

  1. #1

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    How much bellow draw needed for 30" lens for 14xx17

    I would like to do some portraits with my 14x17 which has a max draw of 42". I am thinking about getting a 30" lens but conscerned that I may not have enough bellows draw. Is there a way to determine how much bellows I need for a lens of a specific focal length?

  2. #2

    Join Date
    Nov 2004
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    Re: How much bellow draw needed for 30" lens for 14xx17


  3. #3
    Tin Can's Avatar
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    Dec 2011
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    Re: How much bellow draw needed for 30" lens for 14xx17

    Depends if full body or just head

    I have a life size plastic model that sure helps during setup
    sin eater

  4. #4
    C. D. Keth's Avatar
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    Apr 2006
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    Salt Lake City, UT
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    Re: How much bellow draw needed for 30" lens for 14xx17

    Use the thin lens equation.

    (1/object distance)+(1/image distance)=(1/focal length)

    1/O+1/42=1/30

    Works out to O=105 so you can focus that 30" lens down to 105" (8'9") from the lensboard. That's a comfortable working distance. I wouldn't want to be any closer for a portrait.


    M = (B-F) / F Gives magnification %.

    M=2/5=40% so you can reproduce something just under half size on film. That seems about right for a bust on 14x17 just thinking about and not measuring my head.

    You might want something more like a 19" if you want tight "head shot" portraits and be right about 1:1 with some wiggle room.
    -Chris

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