Originally posted by normhead IN fact, just using my 18-135 DC compared to my DA*60-250, I'd be really reluctant to buy another SDM lens, it's not terrible for speed, but it's also not $1300 great either.
SDM is just an umbrella term for a series of different motors used in different Pentax lenses. There is a good chance new Pentax lenses will have better (and more reliable) SDM than the older ones. In addition to that, this lens will probably have a Tamron SDM.
About the USD (SDM) in the Tamron ePhotozine said:
"As this lens incorporates Tamron's USD silent focusing motor, focusing is reasonably quick and very accurate. Focusing speeds may not be up to the speeds of Nikon and Canon equivalents, but they are not too far behind in all but the darkest conditions."
DPReview said:
"It's pretty quick and responsive, although in side-by-side testing on the same camera bodies it's clearly not quite as lightning-fast as the Canon equivalent. We've seen no obvious systematic problems with focus accuracy on any of the cameras we've used for testing, ranging from the entry-level EOS 650D, through the enthusiast-orientated EOS 60D, to the semi-pro EOS 6D."
So it seems to be reasonably quick, and if it is not far away from Canon and Nikon, it will be at least as fast as the fastest Pentax lenses.