Originally posted by grahame IMHO, F3.2 is fine for this lens. I played with F3.2 on DA21 sometimes, but most time I stop down to 5.6 or smaller.
Color is great, very sharp in the center or most part of the frame. But the biggest issue is the boarders never gets sharp enough even at F8 for city or landscaping.
FA 20 is much better in corners, but still not as good as I thought it would be as an FF lens. I tried FA 20 on A7. FA20 also has to be stopped to 5.6-8.0 to get good performance at corners. Not bad for landscaping, but it might disappoint someone.
i have to say, the lens design has been changed so much in the past decade (or maybe film vs COMS?). Modern lenses are designed to perform better at larger aperture, and older lenses leave bigger space to improve from wide open to optimal aperture.
an other example is FA 501.4 and DA 55/1.4. 55 is much sharper at f1.4 than the older FA 50. At f 1.8-2.0, DA55 is already excellent, stop further to F5.6 improves only slightly; FA 50/1.4 is not comparable at larger aperture, it has to be stopped to 2.8 to get close performance as DA 55. however, at F8.0, my FA 50 is sharper than DA 55.
I used to shot F11 on film, but like to use f2.8 or bigger aperture on DSLR, although I can easier put ISO on SDLR at 800, much more sensitive than 100-200 film I used most. Why? I don't know.
The thick sensor stack on the A7 may be causing field curvature problems with some of these old lenses.
I found better results if I focused using the off center and f11.
I was supposed to send in my A7 for a sensor mod to reduce the filter stack and answer this query on stack thickness on legacy wides, but looks like its a long wait due to supply problems with the replacement filter glass.
As for the original question of DA21ltd or FA20/2.8.
I don't have the DA21, but I have tested the FA20/2.8 on FF.
FA20/2.8 on the A7 can't be a bit better on the edges (last 1/10 of the frame); though like I said above, Sony's choice of a thick sensor stack could be the culprit here.
The pros are a smaller size vs most of its counterparts (1 filter size smaller), good contrast and punchy colors.
The other consideration is that its a FF lens of course.
If TS wants the best, less compromise UWA solution for the new FF, I'd say wait for the new DA UWA zoom which is on the roadmap.
Else, even the cheap Samyang 14/2.8 is better for what it is on FF (only distortion as a con)
But if one wants portability, 20mm and f2.8, the FA20/2.8 is certainly a valid choice imo.