"Rob Studdert" <[EMAIL PROTECTED]> wrote: >USM ring motors are high torque direct drive units >which are fast, quiet, accurate and don't suffer gear/drive train slop like >remote drive systems so are not prone to overshoot if correctly driven. Also >inherent in the design is the fact that as soon as the AC drive is withdrawn >the system effectively becomes de-coupled allowing the lens to be focussed by >hand without the need for additional clutch components.
To my mind, the biggest advantage of USM (or any in-the-lens motor focusing system) is being able to use teleconverters without adding mechanical linkages. -- Mark Roberts Photography and writing www.robertstech.com
