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. Most, but not all, Micro USM lenses from Canon also have full time manual focusing.
Benefits of Ultrasonic motors:
- Faster focusing
- Quieter
- Full time manual focus (for ring-type USM and many but not all Micro USM lenses)
Downsides:
- Higher Cost
Branding
USM is a Canon trademark, so similar terms are used by other manufacturers. These other names include:
- USM: Ultrasonic Motor (Canon)
- SWM: Silent Wave Motor (Nikon)
- SWD: Supersonic Wave Drive Motor (Olympus)
- SDM: Supersonic Drive Motor (Pentax)
- SSM: In-Lens Super-sonic Motor (Sony/Minolta)
- HSM: Hyper-Sonic Motor (Sigma)
- USD: Ultrasonic Silent Drive (Tamron)
