Originally posted by vector I have been comparing both for a while. They are very very similar. The DA of course has better coatings which helps every once in a while. The F has better corners on Full Frame, though I do use the DA on my K1 regardless of the weaker corners. I use the DA more often of the two.
I have been using the DA1.8 and FA1.4. For some reason I never found myself grabbing my F1.7 with the K3 or K1 and it ended up almost permanently attached to my P30T. Nothing wrong with it but I preferred FA1.4 most of the time. The DA1.8 and F1.7 are very similar and I suspect the 1.8 design is just a modified 1.7. I just added the DFA*50 so my FA1.4 is going to feel neglected soon.
I think the 50mm DA 1.8 and the 1.7 models are variations of the same design, but who knows, it could be the same design just restricted to f1.8 instead of f1.7.
But I think they are different... they are both very sharp, the DA at f2.8 has a certain "3D" rendering that the F and other 50mm f1.7 don't seem to have, it's usually the f1.4 versions that have that. So that tells me the field curvature is probably a bit larger with the newer DA, which is actually something Pentax does on purpose to improve rendering in their premium primes.
I'm liking the DA more than I thought I would- the F series coatings are a bit more "cold" and the DA are a bit more warm. Personally I still like the "neutral" M series the best, but this DA lens, at least my copy, makes very pleasing images with excellent micro contrast, in that regard it seems better than the f1.7 designs. Let's see if this is just the honeymoon phase... hopefully I'll still like it after a while, the light weight and the convenience of the SP coatings might make it a 50mm workhorse.