Many people choose to shoot with Pentax camera simply because of the FA ltds line up of lenses. There are three of them; 31, 43, and 77 mm. Some people swear by them, some get hard ons talking about them, and and if you bad mouth'ed them at Pentax Forum you will be dutifully crucified many times over.
I can see how people can get excited about these. This 31/1.8 in particular, along with the 77, is probably the best lens Pentax has ever produced.
A new copy of this goes for close to $1,000. Hm . . . . .
1/30 @ f/1.4