USM lenses are faster.
If you've ever shopped for a new lens for a Canon camera and wondered what some of the letters and numbers on the lens mean, you're not alone. Canon lenses vary widely in price by their performance, and understanding the lens designations can help you decide which lens to buy. One common designation on Canon lenses is USM.
What Is It?
USM stands for ultrasonic motor. A USM lens has a small motor in the lens that assists with autofocus, making the lens focus faster.
Types
Canon actually has two types of USM lens: ring USM lenses and micromotor USM lenses. The two differ substantially in performance and price. Ring USM motors are much quieter and allow for on-the-fly manual focus. Micromotor USM lenses do not allow for manual focus when the camera is in auto-focus mode.
Identification
Both micromotor and ring USM lenses have the USM designation on them, but the specific type is not designated. Check the Canon website or lens catalog and read the specifications to find out.
Performance
Generally, Canon lenses carrying the "L" designation have ring USM. These lenses have a red stripe around the end of the lens. Most of the Canon lenses that have fixed low apertures of f/2.8 also have the ring USM.
Cost
Ring USM lenses are more expensive than micromotor USM lenses, which in turn are more expensive than non-motorized lenses.
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