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  1. #1
    baachitraka's Avatar
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    Diffraction Limit for Xenar 75mm f/3.5

    Camera: Rolleicord Va.

    Using hyper-focal distance with one or two stop(Book: Way Beyond Monochrome) as a safety factor for landscapes, will often see me setting the aperture either to f/16 or to f/22.

    With that I wonder at what aperture the diffraction will start to kick in?

    Also, my print sizes right now are no bigger than 10 inches. If print sizes matter.
    Last edited by baachitraka; 04-24-2013 at 09:06 AM. Click to view previous post history.
    OM-1n: Do I need to own a Leica?
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  2. #2

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    The diffraction limit can be approximated by dividing the f stop into 1500, you will need to know how many line pairs/mm the lens resolves to know when diffraction swamps it. I'd hazard a guess that that lens is diffraction limited somewhere around f:8-11. The only way to be certain (with the lens you have) is to photograph some test charts.

  3. #3

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    I suppose the resolution of the film matters as well, since the meaningful diffraction limit is measured in lp/mm---your film has to be capable of resolving the diffraction before it matters.

    f/8-f/11 sounds too low (wide) to me. If 1500/f is a good rule of thumb, that would imply that the lens has a "native" resolving power of around 150 lpmm...? But maybe I'm misunderstanding something.

    -NT
    Nathan Tenny
    San Diego, CA, USA

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  4. #4
    baachitraka's Avatar
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    I shoot mostly Fomapan 400.
    OM-1n: Do I need to own a Leica?
    Rolleicord Va: Humble.
    Agfa Isolette III: Amazingly simple, yet it produces outstanding negatives.
    Holga 120GFN: EV 11 or EV 12.

  5. #5

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    Won't make much difference on a film that grainy anyway! I don't have a Xenar like yours, but do at times use a 105/3.5 Nikkor M (a very crisp tessar) with a 6x9 film back and high-acutance films like PanF, Efke 25, and Ektar. I don't think I'd generally want to stop down any smaller than f/22. With
    75 or 105 lenses on my Pentax 6x7, even f/22 starts showing a little diffraction with such films. It
    wouldn't start to show unless you're doing a 16x20 print. But ya just gotta test to see what you're
    comfortable with.

  6. #6

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    Quote Originally Posted by ntenny View Post
    I suppose the resolution of the film matters as well, since the meaningful diffraction limit is measured in lp/mm---your film has to be capable of resolving the diffraction before it matters.

    f/8-f/11 sounds too low (wide) to me. If 1500/f is a good rule of thumb, that would imply that the lens has a "native" resolving power of around 150 lpmm...? But maybe I'm misunderstanding something.

    -NT
    You are. The diffraction limit can be approximated by dividing the f stop into 1500, as it depends upon the aperture you're working at. F:2 = 750. f:4 = 375. Diffraction limit means that the lens is well enough corrected that diffraction, not aberrations, are the limiting factor for resolution. It does not mean every f:2 lens resolves 750 lp/mm

  7. #7

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    If the landscape is compelling enough, some diffraction shouldn't matter too much.

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    Quote Originally Posted by BrianShaw View Post
    If the landscape is compelling enough, some diffraction shouldn't matter too much.
    Yes, and with a print size of 10", even f:22 would be just fine. That stop would give a diffraction limit of 69~ lp/mm, and a 4~x enlargement would mean 17~ lp/mm on the print, if the paper could resolve it. Those Xenars are very nice lenses.

  9. #9

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    Quote Originally Posted by E. von Hoegh View Post
    You are. The diffraction limit can be approximated by dividing the f stop into 1500, as it depends upon the aperture you're working at. F:2 = 750. f:4 = 375. Diffraction limit means that the lens is well enough corrected that diffraction, not aberrations, are the limiting factor for resolution. It does not mean every f:2 lens resolves 750 lp/mm
    No, I think I'm not misunderstanding something, then. If the Xenar is diffraction-limited at f/8-f/11, and 1500/f is a decent rule of thumb, then that means that the lens resolves circa 150 lpmm, right? Which sounds kind of high, certainly much finer than most films could resolve.

    -NT
    Nathan Tenny
    San Diego, CA, USA

    The lady of the house has to be a pretty swell sort of person to put up with the annoyance of a photographer.
    -The Little Technical Library, _Developing, Printing, And Enlarging_

  10. #10

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    Quote Originally Posted by ntenny View Post
    No, I think I'm not misunderstanding something, then. If the Xenar is diffraction-limited at f/8-f/11, and 1500/f is a decent rule of thumb, then that means that the lens resolves circa 150 lpmm, right? Which sounds kind of high, certainly much finer than most films could resolve.

    -NT
    136 lp/mm, actually.
    Did you read the part where I said I'd "hazard a guess"? That's what I did, and I've used enough of those lenses on Rolleis to know that they will outresolve some films - for instance Kodachrome PKR 64. 136 lp/mm is not unreasonable for a high quality 75mm lens designed for that format.

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