M any of Canon’s latest lenses use STM (Stepping Motor) systems, which are incredibly quiet and also enable smooth focus transitions when shooting video. Lenses with the lead-screw STM variant can autofocus in almost complete silence, whereas smaller lenses that feature ‘gear type STM’ with gearwheels tend to be more audible and less speedy.
For stills, ring-type USM (Ultrasonic Motor) AF is still often preferred, especially in larger lenses. Again, some small lenses including the Sigma zoom on test use a ‘Micro USM’ variant that has driveshafts and gearwheels. Ring-type systems tend to be faster, quieter and feature full-time manual override in AF mode, whereas this is quite rare with Micro USM systems.