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Ultrasonic Motors (USM)
The Ultrasonic Motor (USM) in Canon EF lenses is the world's first lens-based ultrasonic motor. Based on a totally new technology, the motor spins by ultrasonic oscillation energy. The USM is quiet and quick. It has made EF lenses almost noiseless and autofocusing fast, precise and practical. The direct-drive construction is very simple, with no gear train. This makes it durable and efficient. It also consumes little power. Two types of USM are used: Ring-type USM and Micro USM. The former type is found in large-aperture and super telephoto lenses, while the latter is used in more compact lenses. Using the optimum type of USM in the lens results in maximum efficiency and effectiveness.
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