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Thread: Question about focal length as pertains to minimum focus or default magnification

  1. #1

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    Question about focal length as pertains to minimum focus or default magnification

    Eddie's thread in the FS forum had me asking questions about the minimum focus distance of the Wolly he has for grabs. Some replies were made, but do not satisfy my curiosity, and as I do not wish to further interrupt his for-sale post, I'd like to continue the matter here.

    When I write minimum focus distance, I do not mean the focal length. What I mean is the distance from lens to subject at the focal length extension for a specific LF lens.

    I am new to LF, though I shoot an RB67, and therefore have some experience with increased magnification when bellows are extended beyond infinity.

    It might be best if I describe this in terms of using standard lenses (no bellows/macro/tube modifiers), each lens having a minimum lens-to-subject distance, depending on the specifications of the lens. For example, the old Nikkor-NC Auto 35mm/1.4 that I owned until recently which would close-focus to inches on my Nikon F4s body.

    Now it suddenly occurs to me that the Nikkor only focused to inches on the basis of the internal mechanism for, in essence, extending the distance from rear element to FP.

    If this is correct, then the same could be accomplished if the lens elements within the Nikkor were removed from the housing, made fixed, and a bellows or assortment of tubes replacing the helical within the Nikkor, to obtain the same results. Thus, there is no minimum magnification value for LF lenses beyond the amount of bellows extension available? Would any given LF lense image-circle be exhausted beyond a certain bellows extension, or does the image circle remain constant regardless of extension, provided movements do not cause vignetting?

    OK, so maybe I've no real question after all.

    But, if I am missing some or all of the issue, any further clarification here would be most appreciated.



    C.

  2. #2

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    Re: Question about focal length as pertains to minimum focus or default magnification

    On the image circle bit.

    The image coming out of the back of the lens is basically a triangle shape. The further back the back of the triangle the bigger the image circle. That's why image circle is speced at infinity it's smallest then. At 1:1 it's double IIRC.

  3. #3

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    Re: Question about focal length as pertains to minimum focus or default magnification

    Thank you. So one can only gain coverage by extension. Got it.

  4. #4
    All metric sizes to 24x30 Ole Tjugen's Avatar
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    Re: Question about focal length as pertains to minimum focus or default magnification

    Think of the light coming out of the back of the lens as a cone. Moving the film further back from the lens puts it in a wider part of the cone, so the coverage is bigger.

    There's a pretty basic equation which I get right only by chance, but the thing I get wrong is which letter is what, and it doesn't change anything:

    If the focal length of the lens is F and the distance from the film to the lens is u, then sharp focus will be at distance v from the lens given by the equation

    1/F = 1/u + 1/v

    The scale of reproduction (magnification) will be u/v, so that at infinite distances the scale is 0 since the v is infinitely large.

    What this means is that (without changing the focal length of the lens, like many 35mm lenses do) the minimum focus distance from the lens is equal to the focal length of the lens, but at that distance you need infinite bellows to focus!

    The minimum distance form film to subject is 4xF, which gives 1:1 scale.

    Except for the internal-focus thingies, there is no fundamental difference between LF lenses and 35mm lenses. A neat way to show this is to get an old Leitz Hektor 135mm (for Leica): Remove the outer lens unit from the long tube with the focus mechanism in it, and mount the lens bit on a view camera. That gives you a nice Leitz 135mm barrel lens for your LF camera! You can also mount it on a macro bellows for any 35mm camera, and in many cases you can focus it to infinity and down to about 1:2 (as on my Pentax K100D, with a couple of mount/thread adapters).

  5. #5
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    Tim from Missouri
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    Re: Question about focal length as pertains to minimum focus or default magnification

    HI, again Craig.

    Kodak had a pretty great publication called "Photography with Large Format Cameras" (publication No. oO18 and catalog # CAT 152 7894 that you might still find on ebay or through Amazon.

    It contains some outstanding info on lens function and includes several formulas related to coverage, focal length, depth of field, etc.; as well as camera adjustments, exposure compensation, bellows factors, lens choice for subject type, filtration.....and the list goes on. Bought mine in the early 70's and still find I refer to it a few times a year for refreshers.

    Tim
    "One of the greatest necessities in America is to discover creative solitude." Carl Sandburg

  6. #6

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    Re: Question about focal length as pertains to minimum focus or default magnification

    The comments to this thread have been helpful. Thank you all.

    Best,
    Craig

  7. #7

    Re: Question about focal length as pertains to minimum focus or default magnification

    Craig,

    "Thank you. So one can only gain coverage by extension. Got it."

    and more than a bit of bellows flare, the image circle will be lighting-up more of the bellows. There is a recent thread here about barn-door lens shades, looks interesting.

    Happy New Year.

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