USM stands for ultrasonic motor, a feature available on certain Canon camera lenses. Canon manufactures a variety of lenses for use on its single-lens reflex cameras.
Misconceptions
USM is a feature in the lens of a camera, not in the camera itself. Single-lens reflex cameras have interchangeable lenses, and some of these lenses have the USM feature.
Types
Canon offers two types of USMs: a ring-type USM and a micro USM. The ring-type is used in larger telephoto lenses while the micro USM is featured in smaller wide-angle lenses.
Function
USM allows the lens to focus faster and more quietly than non-USM lenses.
Significance
If you are taking pictures of sports, you need your lens to autofocus quickly or you will miss the picture. A USM lens can help minimize the time between you pushing the autofocus button to the time the lens is focused and ready to take the picture.
Drawbacks
USM lenses are slightly more expensive than non-USM lenses. If you only plan to take still life shots, landscapes or other photos that are not time sensitive, you may be able to save money by buying a lens without USM.
Tags: non-USM lenses, reflex cameras, than non-USM, than non-USM lenses
