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A while back I asked about a cheap fast lens for Nikon cameras, and I've since got myself a nice little 50mm/1.8 AFD. Now I'm looking at doing infrared conversions on the Pentax *istD family of cameras, but I'll need a K-mount lens to use when adjusting and testing the focus. Ideally this lens:

• won't cost too much (I can't see myself using it for anything other than conversions, so I need to be able to cover the cost out of the conversion jobs)
• will have a fairly shallow DOF (the 50mm/1.8 Canon and Nikon lenses are perfect, but I may have to compromise here)
• should have AF so I can check that AF actually does work on the camera!

Does anyone have any suggestions? There are cheap MF lenses around on the 2nd-and market, but the AF requirement does seem to hike up the price...

you can always find a Pentax K mount to M42 adaptor on ebay and use the MF IR spec'dd Takumars. - very cheep.

One of the best kept secret in the Pentax line might be the K-mount 50/1.7. One of the best 50mm lenses ever made, maybe THE best. They are usually not too pricy but can take a while to locate.

if you want auto focus then an F50 1.7 is probably your best bet... it has the infra red focus mark on it.

you can still buy a new FA50 1.4 from B&H for 220 USD or so

Dave, if you're a member of OCAU here's a guy selling a SMC Pentax FA 50mm F1.7 auto-focus lens. Reserve $200 Instant $230.

Thanks for the heads-up on the Overclockers item.
I'm going to see what I can find this weekend at the camera market (in Camberwell) before moving on anything else.

Nikon and Canon definitely have the market share: their 50mm/1.8s are much cheaper (even new) than these Pentax pieces...
But the -F or -FA 50mm/1.7 lenses do look like what I'm after.
Thanks folks!