Pentax have no ring motors, only micro motors. Canon uses USM to refer to ring motors.
SDM does appear from pentax marketing to cover both what we know as sdm (those mechanisms in the da* glass and the da17-70) as well as the different mechanism in the 18-135 refered to as DC. However when we talk on this forum we refer to sdm as different from DC, which they are, even though pentax likes to refer to them both as being a type of sdm.
Maybe we need to talk of SDM-POS, and SDM-DC. But that's too hard, so it's just sdm or dc here. So the question as to whether the 18-135 is SDM I'd say no it isn't, because when people ask that they don't care about what a marketing broucher says, they want to know whether what drives the 18-135 is the same poor excuse of a mechanism that also drives the 16-50 say, and the answer is they are different mechanisms. That matters because DC motors seem reliable thus far, but SDM is anything but.
Last edited by twitch; 06-12-2011 at 06:09 AM.