Doug Kerr
Well-known member
Many lenses in Canon’s “EF” series have autofocus drive systems described as “Ultrasonic Motor” (USM) systems. There are actually a number of different systems in this classification.
I have recently released a new technical article, "The Canon USM (Ultrasonic Motor) Autofocus Drive System" which describes the various forms of this system in considerable detail. Both ring USM and micro USM forms are discussed with respect to both their principles of operation and their deployment in lenses.
Special attention is given to the arrangements for providing the "full-time manual focus" (FTM) feature.
Included is an extensive discussion of the unique AF drive mechanism of the Canon EF 50 mm f/1.4 USM lens, the only EF lens using a micro USM drive system that affords the FTM feature (a matter about which there has been considerable mystery over the years).
The article is directly available here:
http://dougkerr.net/Pumpkin/articles/Canon_USM.pdf
Best regards,
Doug
I have recently released a new technical article, "The Canon USM (Ultrasonic Motor) Autofocus Drive System" which describes the various forms of this system in considerable detail. Both ring USM and micro USM forms are discussed with respect to both their principles of operation and their deployment in lenses.
Special attention is given to the arrangements for providing the "full-time manual focus" (FTM) feature.
Included is an extensive discussion of the unique AF drive mechanism of the Canon EF 50 mm f/1.4 USM lens, the only EF lens using a micro USM drive system that affords the FTM feature (a matter about which there has been considerable mystery over the years).
The article is directly available here:
http://dougkerr.net/Pumpkin/articles/Canon_USM.pdf
Best regards,
Doug