Originally posted by gofour3 If you want a faster lens then get the K50/1.2, that is (and the A50/1.2) are best 50-55mm that Pentax ever made. If you shoot in low light or in the dark then you will appreciate the faster speed and then there is the bokeh!
Phil just reminded me that, in my earlier reply, I hadn't mentioned any 50/1.2's. [Without a whole lot of thinking (LOL), I merely responded to your original mention of the 50/1.4, 50/1.7, 50/2, and 55/1.8 lenses.]
I have had both of the 50/1.2's (the K for a short while and the A for a longer time), and, as Phil has mentioned, they are beautiful lenses, with beautiful, creamy, dreamy bokeh. However:
1. I personally had a fair amount of trouble focusing with them, so that using them wide open (or close to it) and then hitting the focus within the very shallow field was a hit-or-miss proposition for me. [I would assume that not everyone would experience this.]
2. Some users have found them to seem less sharp than other 50's, but that had not been my experience -- I suspect that at least some of these reports were due to difficulties in focusing (see #1). I found that, when I did happen to nail the focus, the lenses were indeed quite sharp (not wide open, of course).
3. They are not cheap.
So, I'll still basically stick with my suggestion, "If I were you (but obviously YMMV), I'd want at least one 50/1.4, at least one 50/1.7, and at least one 55/1.8, and if three 50's were not enough, then [I'd] 'stock up' on the 50/1.4's".
And ~then~, I suppose, you could always spring for a K 50/1.2 later on down the road -- it would ~really~ be ~home~ on that black K2.