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Fred Miranda wrote:
Although I agree that the ZM 25 is superior to the ZM 28, we can't come to definite conclusions just by comparing MTFs. The ZM 28 has stronger field curvature and the weaker MTF probably accounts for that.
The same could be said for the ZF 28/2 Distagon's MTF.
Agreed; but my sense of of the difference wasn't based only on MTFs, but on having thought when playing with the digital M system (I borrowed a bunch of M stuff before getting Sony gear while looking for a FF system for wilderness work that was smaller than Canon FF) that the 25 was astonishingly good, and the 28 fine, but nothing special. But that was pretty informal.
So yes the MTFs might be largely different because of field curvature. But the MTF is at infinity, so if its not like some FC which is mainly at shorter distances and calms down at infinity. If we can mitigate the native field curvature (as against the induced FC) it will be by focussing for a compromise setting -- but that of course, while it gives better results overall, is trading off centre resolution for evenness.
None of this is to say the 28 is not worth pursuing with a PCX, or that it definitely won't turn out to be a good solution. But just that it needs to be pursued in the spirit of experiment, rather than in the confident hope that it'll end up giving great results like the 25 can...
(but here's a wild thought - with these front filters we are basically playing at lens design by adding elements. I wonder if we can simulate what it would take to modify the *native* field curvature by adding an element, effectively doing what Zeiss would have done if their brief/budget/design goal had been different....)
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