Originally posted by Rondec I think that 50mm focal length doesn't work quite as well on a cropped sensor as it does on 35mm. I like lenses either a little shorter (like the DA 40) or a little longer, but each person is different.
Agreed. When I only owned the FA35 and FA50, I was using the 35 the vast majority of the time. Both the DA35/2.4 and the DA40/2.8 are good options if you don't mind the slower aperture and are priced around $200 and $300, respectively.
Originally posted by Rondec The FA 50 is a very nice lens. As dgaies says, it is a little soft wide open and suffers from loss of contrast when there are strong light sources in the frame or near the frame.
The FA 43 and DA 55 are both sharper (and better built), but a lot more expensive.
One thing to remember is that all of the Pentax 50mm f1.4 lenses have the same optical formula, so if you don't mind manual focus, there are an awful lot of options out there for cheap.
I believe the K50 and M50/1.4 are slightly different optically that the A, F and FA50/1.4 (the three of which are the same). Even so, I don't think the difference is significant and as Rondec said you can save a substantial amount over the FA50/1.4 if you don't mind manually focusing. A used K or M50/1.4 usually sell for about $80-120, while the A50/1.4 is typically around $140-180 (depending on condition). FWIW, the M, A, F, and FA50/1.7 all have the same optical formula and you can usually find a used M50/1.7 or A50/1.7 for about $50 or $100, respectively.
EDIT: just noticed in the OP's original post that he (or she) already has the M50/1.4, so you can probably ignore the above paragraph