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Thread: A question for the opticians here.

  1. #1

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    A question for the opticians here.

    I have a small 'pillbox' type lens with a doublet and front mounted washer stops. Its performance is much better close up, and even down to 1:1 than it is at infinity. I believe this is probably as a result of the fixed position of the washer stop. Am I correct in thinking that the washer stop has a specific optimum position relative to the lens itself? Does this actaully vary depending on the focused distance? Is there any useful formula which will help to position the washer stop at any given focus distance? Any other comments?

  2. #2
    Randy's Avatar
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    Re: A question for the opticians here.

    I have no idea if the calculations for the Wollaston landscape lens (meniscus) would help or not. Michael Gasperi, a member here, has a web-page that discuses in detail how to figure the aperture distance for the meniscus lens based on the lens diameter, but I don't believe there is any compensation due to focus distance.

    Anyway, this may help.
    https://dl.dropboxusercontent.com/u/52893762/bigger4b.jpg

  3. #3

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    Re: A question for the opticians here.

    One thing that is probably going on is that at 1:1 you are using only the very center of the field of the lens which is where simple lenses are the best.
    Thanks, but I'd rather just watch:
    Large format: http://flickr.com/michaeldarnton
    Mostly 35mm: http://flickr.com/mdarnton
    You want digital, color, etc?: http://www.flickr.com/photos/stradofear

  4. #4
    Jim Jones's Avatar
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    Re: A question for the opticians here.

    A meniscus lens with a diaphragm in front of the lens as used on many basic cameras from perhaps the 1930s until recently forms the sharpest image on a curved surface. The original box-shaped cameras with the diaphragm behind the lens formed the image of a distant subject on a more flat surface. Thus they are capable of somewhat better overall sharpness.

  5. #5

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    Re: A question for the opticians here.

    never seen a prime lens with an aperture that shifted position relative to the lens elements when focusing, so i doubt that's the issue. most likely the lens elements just aren't optimized for infinity focus, or what mdarnton said.

  6. #6

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    Re: A question for the opticians here.

    Quote Originally Posted by maltfalc View Post
    never seen a prime lens with an aperture that shifted position relative to the lens elements when focusing, so i doubt that's the issue. most likely the lens elements just aren't optimized for infinity focus, or what mdarnton said.
    200/4 MicroNikkor AI/AIS. The aperture is fixed in the barrel, lens elements move to reduce focal length when focusing closer than infinity.

  7. #7

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    Re: A question for the opticians here.

    Quote Originally Posted by Dan Fromm View Post
    200/4 MicroNikkor AI/AIS. The aperture is fixed in the barrel, lens elements move to reduce focal length when focusing closer than infinity.
    I'm sure that this is the case with many of the currently made Internal Focussing macro lenses. I think that most change focal length significantly as they focus closer.

  8. #8

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    Re: A question for the opticians here.

    The diaphragm position has critical influence on many aberrations, and especially on coma. And at least with simple lenses like menisci and achromats, the minimal coma is the criterion of the optimal diaphragm position. Sorry I can't give you any formulae but observing the ground-glass image with a good magnifier, I just move the f-stop back and forth and place it when I like the image best.

    But be careful in choosing your loupe for the calibration. Don't use a 7x magnifier if you want the lens for contact prints only. Better take a 3x or even a 2x one. Simple lenses with lots of aberrations make very fine details (visible only when enlarged) sharp at different distances then the coarser structures important in contact prints and small enlargements.

    And yes coma varies a lot with the lens to subject distance.

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