Originally posted by enoxatnep I'll try to word this in a way that makes sense, but I'm wondering what the differences in optical quality and focus speed are between, say, a DA 40mm and FA 43mm or a DA 70mm and FA 77mm when used on the K20D. I realize the FAs can take great photos on a DSLR, but should one choose the DA versions of similar focal length primes instead because of their optimization for DSLRs? I'm not concerned so much about the extra cost of the FAs if they have better optics and speed on the K20D - I'll pay more for the better options. Hope this makes sense!
I have 3 of the DA Limiteds, and I've read extensive reviews on all the Limiteds (and seen lots of samples), so this is what I've observed: The FAs are about a stop faster than the DAs, have better peak performance especially in the center, and slightly better bokeh. The DAs have a more harmonic performance across all F-stops, and so are sharper wide open and in the corners, but not quite as sharp as the FAs at optimum apertures. The FAs also seem to be more vulnerable to PF/CA, more so with the FA77 and less so with the FA31 (which is as close to optically perfect as perhaps any Pentax lens). If money was no object, the FA31 is a no-brainer on the K20D, and maybe the other two FA's as well. Since I actually own the DA Limiteds, you know I'm not being biased here!