Originally posted by volosong I only have limited experience with Pentax 50mm lenses. My first one is a M f/1.7 that came with my MX body. I still have that lens, and doubt I'd ever part with it. Thirty one years old now...like an old, faithful friend.
I've since acquired...and subsequently sold, two A f/2.0 versions. Was never happy with them and the IQ just couldn't compare to the M f/1.7 lens. Another 50 that I recently acquired is an A f/1.2 version. It is a well-worn copy from KEH and was purchased for bragging rights and for those times when I want to mess around with a very narrow DOF and with existing light photography (in very, very dim lighting situations).
My only autofocus 50 is the FA f/1.4 lens. This is the default one for when I want a 50. So, my comments are based on these above listed lenses...
If I were you, and had the funds available, I'd get the FA50/1.4 lens. It is a fantastic lens for a very reasonable price. I really fear that they will not be available new much longer. Get one now...while you have a chance. Optical quality is among the best that a 50 can give you. It is a very old design, that has been improved as SMC has improved. It autofocuses quickly while also allowing decent manual focus capabilities. However, for manual focus, there is nothing like the old M lenses (and some of the A lenses).
The new DA*55/1.4 SDM is going to be a nice lens, but who knows when it will be available. It also will not have an aperture ring. That might not be important to you, but it is to me for those times when I want to go manual...or even Av (when I don't want to use the body to set the aperture). It will also not be a full-frame lens. The SDM will be nice, but that only help K10D and K100D Super users at this time. We can expect it to be more expensive than the currently available FA lens, and I also highly doubt that the optical quality will be much better than the FA. The FA is pretty good as it is.
I really like manual focusing, as I'm pretty much 'old school'. I've modified my bodies to use the split screen, microprism collar focusing screens. That has greatly aided my old, tired eyes when manually focusing. Spot metering is all screwed up, but that is something I have to adjust for to get the split screen. Perhaps you can't manual focus fast enough because the OEM matte screen 'sux' for manual focusing? The matte screen was designed for auto focus and does not help at all for us who like to manual focus. Something to consider.
Thank you very much. A lot of good advices in your post. I was considering the f/1.7 cos I have the chance to get them at a really good price and I had the understanding that they were slightly sharper than the f/1.4. What I do not know is how they compare to older versions. I guess that the best strategy would be to buy them, test them and then keep the ones that I like the most (thought somebody in my home can kill me).
I will consider also your advice about the split screen. I also like to manual focus when I can but I also like the spot metering when I am shooting in the forest.
Thank you again,