Joe and hcc are quite right. Fast and AF and cheap are mutually exclusive. Fast manual lenses are another matter. In the not-too-distant past I bought a Vivitar-Komine 28/2 CFWA for US$18, a Yashica MF 50/1.4 for $10, a Tamron BBAR 28/2.5 for $4, and a Sears-Tomioka 55/1.4 for TWO BUCKS!! Fast 50s and 28s aren't hard to find nor painfully costly. Those in N/AI, OM, and C/Y mounts are easily modded for Pentax PK; an M42 lens only needs a simple adapter.
Fast MF primes aren't necessarily hard to focus. With my delaminating eyeballs, I depend on Catch-In-Focus (CIF). But longer fast lenses do have very thin DOF. You may want to stay at f/1.7-2 -- and 50/2 lenses are often dirt cheap. To shoot in low light requires some combination of:
* higher ISO - Your Kx is renowned for its high-ISO performance.
* faster lens - My K50/1.2 is a pain to use in the dark. An f/2 is fine.
* firm tripod - You'll be amazed what 30-second exposures can grab.
* more light - Remember, you're photographing light, not objects.
AF often sucks in low light anyway. Go manual!
Last edited by RioRico; 07-17-2011 at 09:37 PM.