Canon's Ultrasonic Motor (USM) was the world's first lens-based motor to harness ultrasonic oscillation energy. Electronic vibrations created by a piezoelectric element power the mechanical action of the lens. This technology makes focusing
precise, virtually noiseless and incredibly quick – some lenses focusing literally faster than the human eye. The USM starts instantaneously and stops the moment focus is achieved, with no overshooting. What's more, the USM requires minimal battery power
so you can shoot for longer on a single charge.
Two types of USM are used in Canon EF lenses: Ring-type USM and Micro USM. Ring-type USM is found in large-aperture and super telephoto lenses, while Micro USM is used in more compact lenses.