types of USM

[reemas]

is there a difference between the USM on canons lenses. i was looking at the 17-85 lens and its a ring type USM. somewhere else i read canon has a USM lens, but it's not a true USM....

can someone clarify?

--sameer

[PacAce]

There's also the micro USM.

[reemas]

whats the difference?

[ron chappel]

All ring types are the good ones :) -fast with the convenience of full time manual overide.

(The EF 50/1.4 doesn't have a ring usm but does allow manual overide-they acheive this by using a normal mini usm motor then running the drive through a planetary gearset which can take input from either the motor or manual focus)

Then there are non ring usm lenses.These are most the consumer zooms that have usm written on them.None of these allow instant manual overide.
Basically they are a mini usm motor running through gears... just like the normal DC motor in standard lenses.
These lenses often come in a standard (non usm) version-there is very little difference in focus speed between the types ,allthough the usm versions are nearly silent.