Canon Lenses with USM?

USM stands for Ultra Sonic Motor. Canon has the micro ultrasonic motor in some of its lenses and it is is a tiny little motor that they stick in the lens and then connect with gears to the focus mechanism. A micro USM does not allow simultaneous AF and MF. Thus, the largest user-interface advantage of the ring USM is gone. Much of the noise of a standard motor is back too. The micro USM is noticeably quieter than a regular motor, but not silent like a ring USM.

Canon puts the micro-USM mostly into its new cheap zoom lenses.

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