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Thread: How to determine focal length of brass lenses from length and diameter measurements?

  1. #11
    8x10, 5x7, 4x5, et al Leigh's Avatar
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    Re: How to determine focal length of brass lenses from length and diameter measuremen

    Quote Originally Posted by Mark Sawyer View Post
    I've never heard of a photographer who knew exactly where his nodal points were for a particular lens, much less use them.
    Absolutely correct*.

    That's why lens data sheets show the Flange Focal Distance, which is from the lensboard to the film regardless of node location.

    With some telephoto designs, the second (rear) node is in front of the front element by many centimeters.

    Quote Originally Posted by Mark Sawyer View Post
    The aperture would be the diameter of the opening of the lens measured through the entrance pupil (front element). Which designs does this not hold true for?
    Correct again, for all lens designs.

    I was clarifying that the dimension used was not the physical diameter of the diaphragm blades at an f-stop setting.

    You can measure the aperture physical diameter with a pair of calipers, but that's not the value needed for the calculation.

    - Leigh

    *NB: One exception is panoramic photography using multiple images.
    For this technique the vertical axis of rotation of the camera must pass through the front (first) node.
    If you believe you can, or you believe you can't... you're right.

  2. #12
    Mark Sawyer's Avatar
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    Re: How to determine focal length of brass lenses from length and diameter measuremen

    Quote Originally Posted by Leigh View Post
    I was clarifying that the dimension used was not the physical diameter of the diaphragm blades at an f-stop setting...
    Actually, you could if you used that f/stop value in the equation. Say you have a lens closed down to f/16, and you measure the opening through the front element to be a half-inch. 16 x 0.5" = an 8 inch lens.
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

  3. #13
    8x10, 5x7, 4x5, et al Leigh's Avatar
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    Re: How to determine focal length of brass lenses from length and diameter measuremen

    Quote Originally Posted by Mark Sawyer View Post
    ...and you measure the opening through the front element... [emphasis mine - Leigh]
    The apparent diameter measured through the front element is the diameter of the "entrance pupil".

    This can be very different from the physical diameter of the aperture blade opening in the shutter with no lens elements mounted.

    This is the distinction I was trying to make.

    The formula is: N = f / D
    where N is the f-number, f is the focal length, and D is the diameter of the entrance pupil.

    The entrance pupil diameter required for a given f-stop is: D = f / N.

    - Leigh
    If you believe you can, or you believe you can't... you're right.

  4. #14
    Mark Sawyer's Avatar
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    Re: How to determine focal length of brass lenses from length and diameter measuremen

    Quite right! I think we're splitting hairs well below the useful tolerances of buying lenses or making photographs, but it's good to keep the working fundamentals precise in our minds.
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

  5. #15

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    Re: How to determine focal length of brass lenses from length and diameter measuremen

    Hello from France !
    The quickest way to determine the focal length of an unknown old lens is to focus on the ground glass the image of a distant object, then measure the distance on the ground glass between two distinct features, make the same experiment with a modern lens of known focal length. The ratio between the 2 focal lengths is equal to the ratio of the distances measured on the ground glass between the same features of a distance object for the 2 lenses.
    This method is valid for any lens, regardless of its lens design, be it a telephoto or whichever asymmetrical lens design for which the distance between the front, middle or rear of the lens barrel and the focal point that can be totally different from the focal length.

    Once you have a good knowledge of the focal length, regarding a practical method of measuring the diameter of the entrance pupil, or estimating the f-number = (focal length) / (diameter f the entrance pupil), this has been discussed here in detail in various posts.
    http://www.largeformatphotography.in...entrance+pupil
    http://www.largeformatphotography.in...entrance+pupil
    http://www.largeformatphotography.in...entrance+pupil
    http://www.largeformatphotography.in...entrance+pupil

  6. #16

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    Re: How to determine focal length of brass lenses from length and diameter measuremen

    I've never heard of a photographer who knew exactly where his nodal points were for a particular lens, much less use them.

    At least, when the photographers we are speaking about, know their focal length, usually they know where the focal point is, except if they only do close-up photography; hence they perfectly know where the rear nodal point is, namely: one focal length ahead of the focal point ;-)
    Eventually, this makes quite a large number of photographers who actually know where their rear nodal point is located.
    As far as the entrance pupil is concerned, agreed, this is a very different story

  7. #17

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    Re: How to determine focal length of brass lenses from length and diameter measuremen

    You got it all wrong! How to Figure Friggin Focal Length and f Stop Thingies:
    http://www.youtube.com/watch?v=CIFNdfjem18

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