Originally posted by y0chang The tamron ultrasonic is a ring motor system. It is as reliable as canon or nikon systems
I wouldn't say that Tamron's ultrasonic AF is as precise or reliable as Nikon's SWM or Canon's USM. It's about on par with Sigma's HSM, which is to say, it's hit-or-miss at times, but generally very good.
I haven't yet tested Pentax' SDM AF, I've only ever used DC lenses I think.
However, it seems to me that Pentax is doing a bit more with this lens, hopefully. For example if they're putting the WR badge on it, they might just be weather sealing the lens a bit more than Tamron did originally.
All in all, I would expect it to be BETTER sealed than a Canikon lens, but only 75-90% as good at AF. For most, that's 100% as good as they'll ever need, of course.
Personally, I'm very excited. The optics of the existing Tamron are definitely good enough for crucial work, even at 2.8, and if the Tamron 15-30 is also in the pipeline for Pentax, that's even better news for me as an astro-landscape photographer. Now I have the 15-30 2.8 when I need zoom range but don't mind the 2+LB weight, or the Rokinon 14mm f/2.8 in K-mount when I need ultrawide but without breaking my back. Yay! Now if only Pentax can do a 24 1.4 FA WR, or a ~21 1.8 FA Limited, I'd be in full-frame astro-landscape heaven...
=Matt=