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Thread: ILEX-Caltar 508mm 20" Lens

  1. #11

    ILEX-Caltar 508mm 20" Lens



    Questions like "How close can I focus with this lens on my camera (with max bellows extension X)?", or "What magnification will this lens give at extension Y?" are easily answered with basic optical equations. One source for the equations is the Lens Tutorial at http://www.photo.net/learn/optics/lensTutorial. The equations are in the first section, "Object distance, image distance, and magnification" Be sure to use the same units (e.g., mm) for all distances and the focal length.


  2. #12

    Join Date
    May 2004
    Posts
    7

    ILEX-Caltar 508mm 20" Lens

    Michael, thanks for the "Lens Tutorial" link, but beside bringing back nightmares of math exams that I didn't study for, I'm also not sure that the formulas given really address my question.

    ERNEST!!!! HELP!!! You seem to be very good at putting complicated things into simple terms. So, once again here's my question:

    If, with my 4x5 camera I use a 240mm lens at "X" camera-to-subject distance in order get the image size I want on the ground glass -- with a 50% bellows extension (ie - 360mm) -- would I be correct to assume that a 480mm lens on an 8x10 camera, at the same camera-to-subject distance as with the 4x5, would result in proportionally the same image size on the ground glass.... with 50% bellows extension (ie -720mm) ?

    In other words, I just want to know, if I will get the same image on the ground glass of an 8x10 by doubling the lens focal length that I used on the 4x5. Simple.

  3. #13

    ILEX-Caltar 508mm 20" Lens

    Yes, to convert 4x5 focal lengths to equivalent focal lengths for 8x10, just double them.

    If you could someone take your 4x5 camera and magically double _every_ dimension -- focal length, ground glass size, distance of lens from film -- the amount of the scence visible on the ground glass would be the same. The reason for this is that doubling all of the linear dimensions doesn't change any of the angles. This is geometry -- it would work with a pinhole lens.

  4. #14

    Join Date
    May 2004
    Posts
    7

    ILEX-Caltar 508mm 20" Lens

    Michael... THANK YOU! Now that's what I call a simple, clear and definitive answer. I can sleep soundly now.

    Thank you all for your input and ideas.

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