What is USM, and what are its pros and cons?
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
- Full time manual focus (for ring-type USM and many but not all Micro USM lenses)
- Higher Cost
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)
another downside: weight. Another name for USM Nikon uses is AF-S (Auto Focus - Silent wave).
"...The preferred type is "Ring Type" which always allows for manual focus without turning off auto-focus." Can you provide an example of a Canon lens that includes a micro-USM motor that does not have full time manual focus? I've not come across one that doesn't. The EF 50mm f/1.4 has a micro-USM motor and certainly has full time manual focus.
@MichaelClark: EF 75-300mm USM (non-IS) series has micro-USM but not full-time manual focus.
The EF 75-300mm f/4-5.6 USM III is not a lens, it is a paperweight pretending to be a lens. :-)
OK, so there is at least one Micro USM lens that does not have FTM focus, but the majority of Micro USM lenses *do* and that should probably be reflected in your answer.