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Thread: Focal Length Confusionism

  1. #1
    Unwitting Thread Killer Ari's Avatar
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    Focal Length Confusionism

    I may have blacked out and not fully regained my senses, but something is puzzling me.
    When I put an old Paragon 12" lens on the camera and focus at infinity, the distance from lens board to film plane is 10".
    Similarly, on a newly-acquired Wollensak Vesta 14", the bellows are at 10" when I'm focused at infinity.
    Only a 12" Cooke lens lines up with 12" bellows, my other lenses use shorter bellows than the FL indicates.
    Am I missing something?
    Thanks

  2. #2

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    Re: Focal Length Confusionism

    Probably not.

    Flange-to-film distance given focal length depends on the lens design. For example, my 101/4.5 Ektar, a tessar type, has a larger f-to-f than my 105/3.7 Ektar, a Heliar type. To add to the fun, lenses in barrel have f-to-f distances that are only loosely related to focal length. For example, I have a 150/6.3 CZJ Tessar in a deep mount whose mounting surface is nearly at the front of the barrel.

    For lenses of normal construction -- non-tele, non-retrofocus -- diaphragm-to-film distance is usually approximately focal length, but not always. 6/4 double Gauss types are often somewhat telephoto, i.e., sit a bit closer to the film than the focal length would suggest. This is why many 1950s and early '60s "normal" lenses for 35 mm SLRs have 58 mm focal lengths. Designing a 50 mm 6/4 double Gauss type with adequate performance and enough back focus to work in an SLR took more time than was available ...

  3. #3
    Mark Sawyer's Avatar
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    Re: Focal Length Confusionism

    Quote Originally Posted by Ari View Post
    ...the distance from lens board to film plane is 10".
    That might be the problem. Measure from the aperture's location, not the lens board's.
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

  4. #4
    Drew Bedo's Avatar
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    Re: Focal Length Confusionism

    Ah yes . . .The Nodes.
    Drew Bedo
    www.quietlightphoto.com
    http://www.artsyhome.com/author/drew-bedo




    There are only three types of mounting flanges; too big, too small and wrong thread!

  5. #5
    ic-racer's Avatar
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    Re: Focal Length Confusionism

    Focal length is from rear nodal point to image at infinity. f' in the image below.
    The issue you are having is that the rear nodal point is not usually marked on large format lens barrels, shutters or lens boards.
    Click image for larger version. 

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  6. #6

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    Re: Focal Length Confusionism

    Quote Originally Posted by Drew Bedo View Post
    Ah yes . . .The Nodes.
    Isn't there a node thread in the Image Sharing and Discussion section?

  7. #7
    Unwitting Thread Killer Ari's Avatar
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    Re: Focal Length Confusionism

    Quote Originally Posted by Mark Sawyer View Post
    That might be the problem. Measure from the aperture's location, not the lens board's.
    Yes, of course you're right. Thanks, Mark.

  8. #8
    Mark Sawyer's Avatar
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    Re: Focal Length Confusionism

    Quote Originally Posted by ic-racer View Post
    Focal length is from rear nodal point to image at infinity. f' in the image below.
    The issue you are having is that the rear nodal point is not usually marked on large format lens barrels, shutters or lens boards.
    Click image for larger version. 

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    Focal length is from the rear node to the focal plane at ANY distance. Infinity is just a standard distance to measure at, hence the bellows extension factor for close ups; you've changed the focal length of the lens by focusing closer. Quite right that the rear nodal point is seldom marked, but it's generally pretty close to where the aperture is (exactly where the aperture goes is one of the "design freedoms"), so close enough for all practical purposes.
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

  9. #9

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    Re: Focal Length Confusionism

    Son-of-a-gun! The OP asked why flange-to-film distances of lenses with similar focal lengths aren't much the same and was distracted into accepting explanations based on invisible points.

  10. #10
    Unwitting Thread Killer Ari's Avatar
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    Re: Focal Length Confusionism

    I wasn't distracted, I was just...hey, what's that shiny thing there?!

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