My Canon lenses have the designation USM on them, which I assume is for the same reason that they say "Ultrasonic". What does this mean, and why or why do I not want it?
USM - Ultrasonic motor (This is the Canon Terminology)
This is a big improvement over older micro-motor based autofocus systems, which are significantly slower and louder. There are two types of USM systems "Micro" and "Ring". The preferred type is "Ring Type" which always allows for manual focus without turning off auto-focus.
Benefits of Ultrasonic motors:
USM is a Canon trademark, so similar terms are used by other manufacturers. These other names include:
You want it for a few reason:
You may NOT want it because:
Call me dumb but after few minutes of hard-thinking I still cannot come up with ANY other reason you may NOT want it. Making my answer almost identical to the previous one lol.