As per some of the previous commenters, I've had excellent experiences with Pentax 50mm lenses. My favourite was probably a screwmount Super Takumar 50mm f1.4, but I still have a Pentax-A and Pentax-M 50mm f1.7s, and a Pentax-M 50mm f1.4 and they are all good. I think the f1.7s are slightly sharper than the f1.4, but they are all good.