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ultrasonic focus motor

Abbreviated as USM, and associated with digital camera terminology, ultrasonic motor (also called ultrasonic focus motor ) is a term that is mainly used to describe the motor that controls the movement of the lens or focus ring. An ultrasonic focus motor is much quieter (almost near-silent) and also moves the focus lens or ring faster when compared to a standard motor. A USM is able to hold the lens in place with precision and accuracy at the exact instant the lens arrives at the correct focus when using the camera's auto focus (AF) feature.

See "Digital Cameras" in the  Quick Reference section of Webopedia







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