What's the difference between them? Traditional motors such as afd/mm convert electromagnetic force into rotating force, while usm is driven by guided waves based on the energy of ultrasonic vibration.
There are two different types of usm:
Ring usm: Ring usm is the fastest usm. Usually accompanied by full-time manual (ftm) function.
Micro usm: Micro usm is used for cheaper Canon lenses, such as 75-300ii, 28-80ii-iv, 35-80 and so on.
Canon 50/ 1.4usm is the only lens with ftm function in a micromotor!
Usm has the following characteristics: Quiet! All you can hear is the sound of the focus lens group moving. The annular usm is much faster than the traditional focusing system.
Micro usm is a little slower than ring usm, but it is still quite fast compared with AFD/mm, with low energy consumption. There is no problem at extreme temperatures. Afd/mm: high noise.
Unless it is a built-in focusing lens, it is quite slow. Afd is a little faster than mm, and it consumes a lot of energy.