I would say spring $40 to $60 or whatever for an A 50mm/2 or an A 50mm/1.7.
I have an older DS, but it is with that same 18-55mm lens. It is a good lens, useful and efficient. It is not brilliant. But I would never get rid of it, like
RioRico said. A month or two back I rooted out some old 70s/80s vintage primes I had and wow, what a difference. I am kicking myself for wasting several years using the kit lens and 80-320 FA zoom! I got very frustrated of always having only "average" photos when I knew I should be getting better results. There may be some zooms that can equal the optical quality of those old M, K, and A lenses, but I can't afford them. Get a 28 or 35mm if you want something with less magnification than the 50mm, but I find the 50mm units very workable as a "standard" lens even on digital. And they are cheap and plentiful. There is a lot to be said for cheap and plentiful. If only you could get _____ cheaply and plentifully! (insert whatever you would like in the blank. Maybe we should have a fill-in-the-blank contest!)
If you want "reach," I'd say keep using the kit, and get a 135mm fixed length. I have seen some stunning deals on these if not Pentax brand - there are many decent though not brilliant third part 135mm fixed lengths ("primes") going back decades. I had an M 135/3.5 back in the 80s and it was a very useful lens (I don't know what happened to it!). Keep in mind that is more like 200mm when on digital if you still think in film magnification like I do. My little "go" pack is a modest fanny pack with a 50mm, the kit, a 135mm and flash. By far the 50mm is the 85% of the time lens now. I am installing a split/matte old-school focus ring next month. I am going to grind my own from a vintage ME of mine that died a noble death. You can buy an inexpensive one from China via Ebay for $20 to $30 and it will make your manual-focus life easier. I'm just going to grind down my own to both a) save $30, and b) salvage some part of that old ME; it will be very satisfying to have some late '70s vintage parts now "improving" my DSLR. I saw how to do it from some excellent posts by forum members here - which is one of many reasons I paid to be a "supporter."
But by far the biggest leap for me was taking off the kit and sticking on the A 50/2. Wow what a jump in image quality and my personal satisfaction. Keep in mind if your Kx is like the DS, there are a few tweaks to using it off the "A" setting. I also agree with RioRico's point and if I could afford an FA 50/1.4, that would be my everyday lens. Maybe in a few years. I am thinking I will pick up an A 50/1.4 when I find a good deal. I have not used an FA 50/1.4 but boy would I like too!
Last edited by Oro; 08-28-2010 at 12:15 AM.