I just stop down as far as needed for the shot. I don't really care about diffraction.
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?
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.
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
Ntenny, here's another on combined film/lens resolution. See pp. 8. http://vitaleartconservation.com/PDF..._v3a_draft.pdf
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.
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.
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.
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