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You can't go far wrong with any of the SMC Pentax 24mm lenses. The best of the lot is the autofocus SMC Pentax-FA 24/2. I sold one on eBay last year for $590 US. The SMC Pentax 24/2.8, SMC Pentax-A 24/2.8, and SMC Pentax 24/3.5 are also quite good. I owned the SMC Pentax-K 24/2.8, which I found to be better than the Nikkor 24/2.8 AIS, Olympus H.Zuiko 24/2.8 MC Auto-W, and Sigma Super Wide II 24/2.8. It was about equivalent to the Canon EF 24/2.8 (a very nice little lens), which I kept, as I'm an EOS shooter. I haven't owned the SMC Pentax 24/3.5, but I did own the very similar SMC Pentax 28/3.5 and 35/3.5. They were both very small, and very sharp. The SMC Pentax 24/2.8 and -A 24/2.8 are also small and sharp. In comparison, the -FA 24/2 is quite large.
Boz Dimitrov's site http://www.bdimitrov.de/kmp/lenses/index.html is the best resource on Pentax lens specs and technology. Stan Halpin's site http://stans-photography.info/ offers information/reviews on Pentax lenses.
I've attached below the "24mm" section from an older resource site that's no longer available online; Alex's BOA Gallery.
Quoted from BOA's Gallery: Pentax Lenses Review, http://home.att.net/~alnem/html/equipment_review.html
Last Updated on Monday, November 08, 1999
K and A 24/2.8's are optically identical. Pentax K24/2.8 is a great lens with very few distortion, high resolution, great color rendition, and good color saturation.
There's no M24/2.8.
A 24/2.8 is very resistant to flare and ghosting, very sharp when stopped down. A 24/2.8 is reputed to be better than M24-35/3.5 and M24-50. Comparing K(A)24/2.8 and K24/3.5, K(A)24/2.8 has higher center resolution/sharpness and lower corner resolution/sharpness. Both of them are excellent performers.
FA24/2 AL IF is said to be an excellent lens, and optically better than both K(A)24/2.8 and K24/3.5. FA version is very well build and exhibits excellent contrast and sharpness. Its handling is very good, especially when used as a manual focus lens due to it's focus clutch mechanism whereby you pull back on the focusing ring, which now has a relatively nice manual focus feel.
I've also owned the SMC Pentax-M 24-35/3.5 mentioned above, and I found that the SMC Pentax 24/2.8 was better.