Originally posted by normhead MY understanding is SDM is a small electric motor that drives a gear, like screw drive with an internal motor.
SDM just means the motor is a piezo motor. It can be small and weak with gears (like most old Pentax designs and the recent Nikkor 105/1.4 ("AF-S") and the still current Canon 50mm F1.4 ("USM")) or it can be large all around the lens ("ring motor") like the Tamron/Pentax 24-70/2.8.
Originally posted by Mistral75 DC is not a ring motor, it's a micromotor, and so are the non-Tamron SDMs -until now, that is
Same here. DC in most older designs from Sigma, Pentax etc is a very traditional motor with gears. But it can also be large and around the lens linear motor like presumably in the DFA*70-200 and DFA 150-450 and what is used in record players and washing machines.
The abbreviations hide very different designs.
The large motors provide much more torque and allow quicker and more precise refocussing.