USM
In order to achieve critical autofocus, the elements within a camera lens have to move quickly, quietly, and with exacting precision. To accomplish this, Canon developed the world's first lens-based Ultrasonic Motor (USM). Based on a totally new technology, the motor spins by ultrasonic oscillation energy. In effect, instead of a large, noisy drive-train system, electronic vibrations created by a piezoelectric ceramic element power the mechanical action of the lens. In a very broad comparison, it's like the difference between a mechanical watch with gears and springs and quartz-timed watch movement. Therefore, torque is constant and start and stop functions virtually instantaneous.
By making the autofocus operation almost inaudible and by accelerating the autofocus speed, this type of motor greatly expands the shooting possibilities for Canon photographers. There is virtually no whining noise as the lens focuses to distract the subject or draw attention to the photographer. Most important for the photographer in the field, the motor draws minimal power so the camera batteries last longer.
There are two types of USM Focus systems Micro and Ring-type.
Ring-type USM-equipped lenses, found in large aperture and super telephoto designs, allow manual focusing without switching out of the auto mode.
Micro USM designs bring the performance benefits of Canon's USM technology to a wide assortment of affordable EF lenses. Extremely compact and light.
