by Philip Greenspun; created 1996
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.
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.
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 EOS 5 body).
This is the "right stuff."
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.
Text and pictures (c) Copyright 1991-1996 Philip Greenspun