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Canon was very smart and put an AF motor in each of their EOS lenses. This
means that you always have the correct size motor for your lens and therefore AF
will be fast. There are four types of Canon AF motors.
Arc Form Drive (AFD)
This was the original Canon EOS motor. It is relatively noisy and slow. It
does not allow simultaneous auto and manual focus. You do not really want a lens
with this motor. No new Canon EOS lenses are being designed with this motor, but
they are still selling some older designs with the AFD motor. As of November
1996, some lenses with AFD motors include the 50 macros, the 24, 28, and 35 fixed
wide angles, and 135 soft focus lens, and the 100-300L zoom.
An even cheaper and crummier motor is the "micro motor", available on the 100
macro, 50/1.8, and two really cheap zoom lenses.
Ring Ultrasonic Motor
Beginning in 1987 with the 300/2.8, Canon has been slowly equipping its best
lenses with the ring ultrasonic motor, in which the rotor and stator are big
circles around the optics. These are incredibly fast and silent. Even more
important, they allow simultaneous use of autofocus and manual focus. You can
leave the lens in AF mode all the time and override the camera's decision at any
time (or move AF to a separate button as with the
This is the "right stuff."
Micro Ultrasonic Motor
If you were a big company and figured out how to make something for 1/3rd the
cost but still sold under the same name, wouldn't you? Well, that's what Canon
did. The micro ultrasonic motor is a tiny little motor that they stick in the
lens and then connect with gears to the focus mechanism. It is tough to see how
this is any better than the Minolta/Nikon AF method of driving the lens focus
mechanism from a motor inside the lens mount.
A micro USM does not allow simultaneous AF and MF. Thus, the largest
user-interface advantage of the ring USM is gone. Much of the noise of a standard
motor is back too. The micro USM is noticeably quieter than a regular motor, but
not silent like a ring USM.
Canon puts the micro-USM mostly into its new cheap zoom lenses.