Originally posted by wmmk sorry to sound like a n00b, but what is SDM?
No worries.
SDM = Supersonic Direct-drive Motor
It's a motor in the lens that drives the focusing mechanism, much like the Supersonic-Wave Motor (SWM) in recent Nikon lenses, or UltraSonic Motor (USM) for Canon.
Older lenses would have the focus driven by a screw in the body. On the K mount, you will see the drive screw at around the 5:00 position.
What the SDM hopes to achieve is quicker focus times, and quieter focus mechanisms. As you may already know the body driven focus motors are noisier. Try a kit 18-55 (uses SWM) on a Nikon D50 or D70, then bolt on an older screw driven 75-300 on and you'll hear what I mean.