Quote Originally Posted by ntenny View Post
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
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