Toralf Lund wrote: >>On the low-end, AFD was replaceed by the micro-motor drive, which is >>faster with small, light lenses but not powerful enough for anything >>heavy. It's essentially a small high-speed motor. >> > > It's essentially a normal DC motor with an ironless core, isn't it? (Or > am I referring to something else?) > > This is surely technology one might equally well use in an in-body motor.
Correct, the major advantage here is the ability to match the motor to the load. Note this doesn't actually perform any better than a powerful in-body drive, given equivalent AF sensor and logic. > >>Then Canon introduced the USM Ken describes, with the focus ring being >>an electronic control (which was also the case in certain early AFD >>lenses, a couple even lacked MF capability entirely)meaning no focusing >>without power. These are the lenses that made Canon's reputation for >>fast AF. They would later be replaced by 2 different types of USM. The >>first is Ring-Type USM, which adds a clutch mechanism for mechanical >>full-time manual focus and works faster, it's essentially an improved >>version of the original USM, these lneses also provide distance info for >>flash exposure. The second is called micro-motor USM, and is merely a >>conventional micro-motor driven by ultrasonic AC. > > Surely that does not make it an "ultrasonic motor"? I mean, doesn't the > term "ultrasonic" usually refer to (acoustic) vibrations that are > actually driving the motor? Applied to an AC voltage it seems rather > meaningless... But it IS driven by ultrasonic frequency AC. So it's still an Ultrasonic Motor, and still silent, just not nearly as fast. > > Apart from that, thanks for an interesting overview. > > - Toralf > -Adam -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net
