Diffraction Limit for Xenar 75mm f/3.5

Discussion in 'Medium Format Cameras and Accessories' started by baachitraka, Apr 24, 2013.

  1. baachitraka

    baachitraka Subscriber

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    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 a moderator: Apr 24, 2013
  2. E. von Hoegh

    E. von Hoegh Member

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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. ntenny

    ntenny Member

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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
     
  4. baachitraka

    baachitraka Subscriber

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    I shoot mostly Fomapan 400.
     
  5. DREW WILEY

    DREW WILEY Member

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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. E. von Hoegh

    E. von Hoegh Member

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    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. BrianShaw

    BrianShaw Member

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

    E. von Hoegh Member

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    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.:smile:
     
  9. ntenny

    ntenny Member

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    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
     
  10. E. von Hoegh

    E. von Hoegh Member

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    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.
     
  11. BrianShaw

    BrianShaw Member

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    I have a lot of experience with that Xenar on a Vb... and have never noticed a diffraction "problem" even when using very small apertures for small (8x10) prints, or even 11x14 when viewed from a "normal" viewing distance.
     
  12. cjbecker

    cjbecker Member

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    I just stop down as far as needed for the shot. I don't really care about diffraction.
     
  13. ntenny

    ntenny Member

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    Yeah, I hope we're not fighting about this or anything, I'm just surprised that your hazarded guess would be as wide as that.

    Chris Perez's lens tests, for instance (http://www.hevanet.com/cperez/MF_testing.html), found almost nothing that could deliver more than 100 lpmm to the film; one very very good Xenotar, a couple of Mamiya lenses, and that's about it. The Tessar he tested peaked at 68 lp/mm (that includes the film's MTF, though).

    That said, it's not totally clear to me if the diffraction limit is the sole driving factor for the sharpest aperture, or if there are other issues to consider---and most of the lenses, including the Tessar, in that test have their sharpest results at the f/8-f/11 you guessed, but at delivered resolutions far below that theoretical 136 lp/mm (or 187.5 lp/mm at f/8).

    Is this confusion I have making any sense? It seems like, on the one hand, hitting the diffraction limit at f/8 or f/11 would require rather stellar performance from the lens, per the numbers above. On the other hand, the Tessar Perez tested (to pick one example; the other lenses do broadly similar things) starts to show dropoff at apertures above f/8-f/11, but it's doing it while delivering resolutions *far* below what the diffraction numbers would suggest---60-68 lp/mm in the case of that lens. You'd expect some loss due to convolution with the MTF of the film, and some due to in-camera issues like film flatness, but are those components really likely to explain all of the difference between 136 and 60 lp/mm, or are there other effects to be considered here?

    I'm not trying to argue about anything, just trying to understand how to reconcile these different numbers I'm seeing. Thoughts?

    Thanks

    -NT
     
  14. Matus Kalisky

    Matus Kalisky Member

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    Lenses are (were) usually designed to reach optimal performance at (from) certain f/stop - because different optical aberrations are affected differently when 'stopping down'. Good example are many large format lenses where the 'wide open' aperture (anywhere down to f/9 depending on the lens) gets only used for focusing and one stops down to at least f/16 to get the best optical performance. Of course - with large format one in general does not use large enlargement factors so even f/45 (or less with 8x10" or bigger) is not a problem.

    Tesar lenses were/are known for very good performance on axis even at larger aperture, but require stopping down (my experience with Rolleiflex T says at least to f/8) to improve the corners.

    I am not going to dip into diffraction, because whether one sees it in the final print depends on several factors and it would probably be easier to simply test it than discuss it to death. The DOF calculators (+ the knowledge of the dpi in the printing) on the web give a good starting point. But I would not expect diffraction problem down to f/22 with 10" prints from 6x6.
     
  15. E. von Hoegh

    E. von Hoegh Member

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    Testing the combination of film and lens doesn't really always give much info about what the lens is ideally capable of. I was basing my guess on where the lens would become limited, not what the lens/film combo would do. If you have a lens capable of 120 lp/mm, and use perfectly flat film with the same resolution, you won't get 120 lp/mm on the film . But, if you used the same lens with film able to resolve say 800 or 1000 lp/mm, you'd get a much closer estimate of what the lens would do. IIRC, Perez used T-Max 100 for the tests which is a sharp film, and his tests give a pretty good practical guide to what one can expect with those lenses in the field.

    From one of C.Perez's articles:"He does conclude with a table showing the resulting combined lens/film
    >resolutions from f2.8 to f22. This table assumes ideal, diffraction
    >limited lenses (not the actual measured aerial resolution numbers sited
    >in his previous table), and a film capable of resolving 120 lpmm (of
    >course, there are few if any films capable of 120 lpmm for subjects of
    >"normal contrast, but since most measurements use test charts of fairly
    >high resolution, I am willing to accept the 120 lpmm for test charts and
    >scale down for normal subjects of lower contrast).
    >
    >Here's his final table (assuming ideal, diffraction limied lenses):
    >
    >f-stop combined lens/film resolutionI Itheoretical max. lens resolution
    >f2.8 80 lpmm----------------------------------600 lpmm
    >f4 90 lpmm 400 lpmm
    >f5.6 95 lpmm 300 lpmm
    >f8 100 lpmm 200 lpmm
    >f11 95 lpmm 150 lpmm
    >f16 80 lpmm 100 lpmm
    >f22 70 lpmm 75 lpmm
    >
    >So, even assuming an ideal diffraction limited lens combined with a high
    >contrast subject and a film capable of 120 lpmm, he concludes the best
    >possible on film resolution will be 100 lpmm at f8 (BTW, this conculsion
    >is independent of film format, since he is using theoretical,
    >diffraction limited lenses). Again these are his conclusions, and I am
    >merely quoting them here (under "fair use"). For copyright reasons, I
    >have not copied the entire article. Also, like I said above, I don't
    >fully understand the relationship between aperture and film resolving
    >power. Although I don't dispute his numbers in anyway, I would like to
    >understand their derivation better"

    The whole article is here - http://www.hevanet.com/cperez/results.html
     
  16. E. von Hoegh

    E. von Hoegh Member

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  17. DREW WILEY

    DREW WILEY Member

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    120 film is cheap enough. Just do a test at each aperture then print it. You'll learn ten times more in
    five minutes than spending two hundred years with a calculator trying to figure this out.
     
  18. ntenny

    ntenny Member

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    Apart from the intellectual interest of trying to understand how all the elements of the imaging chain affect the final outcome, the important message perhaps is that "Where's the diffraction limit?" is not as useful a question as "Where's the sharpest aperture?", whether the falloff in sharpness at smaller apertures is due to diffraction or to other effects or a combination. The empirical data amassed by Perez and others certainly suggest that most MF lenses, including the Tessars and Xenars used on Rolleis, peak in resolution around f/8-f/11. But the drop in resolution at f/16 isn't very dramatic, and a lot of practical user experience suggests that the actual visible effects are pretty minimal up to f/22 if not beyond.

    Thanks for the pointers to Perez's article and others.

    -NT
     
  19. Henning Serger

    Henning Serger Member

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    Hello neighbour (greetings from Lower-Saxony :wink:),

    probably I can help giving some data:
    The Xenar is a 4 element Tessar design.
    The Yashinon 3,5/80 in the Yashicamat 124G is also the (same) 4 element Tessar design.
    We've tested the Yashinon 3,5/80 in our optic lab.

    This lens has its best performance at f8.
    With an object contrast of 1:4 (two stops) we achieved 80 clearly separated linepairs per millimeter on Provia 100F.
    The resolution at f11 was only minimal lower. The difference can only be seen at 100x enlargement under the microscope.
    For normal daily photography the difference is irrelevant.
    At f16 there is a visible resolution loss due to diffraction, and of course even worse at f22.

    But as you say your print sizes are now not bigger than 10 inches (which is a very small enlargement factor for 6x6), using f16 would be no problem.
    Depending on your requirements concerning quality, maybe even f22 will work (well for me it does not work well, I stop down max. to f16).
    Just try it and look if you like it.

    Best regards,
    Henning
     
  20. Henning Serger

    Henning Serger Member

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    Hello Nathan,

    I've great respect for the work done by C. Perez, but nevertheless I have to disagree concerning the statement that it is so seldom to get system resolution values of lens / film combinations delivering more than 100 lp/mm.
    I have to disagree because of our own test results in our little non-profit optic test lab.

    The bottleneck for getting high resolution is less the lens and film, it is
    - absolutely precise focusing
    - avoiding of all vibrations.

    To get the optimum detail rendition the focus has to be absolutely spot on. You don't have any tolerance if you want best performance.
    Therefore focus bracketing is needed in resolution tests. And Perez didn't use this technique.
    If he had used it, his test results have been significantly better with higher values.
    Focusing in medium format cameras with matte screen or split-image is not very accurate. You can try yourself: Make 15 shots of the same subject, each shot newly focused, and then compare all these shots at big enlargements.
    You will see that you don't have 15 identically sharp shots, but several different sharpness and resolution levels. You probably even will have one or two almost unsharp shots, and most probably 2-3 really sharp shots with exact, precise focus.

    Some test results from our test series (all with object contrast of 1:4, two stops):
    Mamiya 645 pro TL with Sekor C 2,8/80 N at f5,6:
    Provia 100F: 115 - 125 lp/mm
    Agfa Copex Rapid: 130 - 145 lp/mm
    Adox CMS 20 II: 195 - 210 lp/mm

    Best regards,
    Henning