Ring vs Micro USM - How not to get taken by marketing hype

Started Oct 2, 2005 | Discussions thread
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Steve H
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Ring vs Micro USM - How not to get taken by marketing hype
Oct 2, 2005

The term "USM" stands for "UltraSonic Motor" - a type of auto-focus drive fitted to many Canon lenses. Although the term USM is quite well known, less well known is the fact that there are actually two completely different types of USM - one good and the other probably not worth worrying about.

The original and best USM, known as a "ring type USM", was introduced by Canon in the early 1990's. This type doesn't even look like a motor and instead appears to be nothing more than two large rings with a few wires poking out of them. One ring is connected to the focusing parts of the lens and the other to the barrel, and when one part is made to vibrate at ultrasonic frequencies (hence the name) this translates into rotational movement of the other. This type is fast, almost completely silent (except to dogs and bats!) and there is no need for a gear train or other noisy mechanicals. Ring USM also has another advantage - full-time manual focussing or FTM in which, once auto-focus has completed, the lens focus can be "touched up" using the manual focus ring without having to switch the lens from AF to MF. Canon uses ring type USMs on nearly all its mid-range and L-series lenses except for a few very early designs which were introduced before USM was invented.

Ring type USMs:

The other (and lesser) type USM is known as a "micro-USM". This is a form of USM motor that Canon designed for their less expensive lenses so they can bill them as "ultrasonic" - essentially for marketing purposes and to cash in on the reputation of the ring-USM. These little motors operate on the USM principle but still use a noisy mechanical gear train and so are really no different or better than their cheaper micro-motor alternative. They also (except for the 50/1.4 and 28-105/4-5.6) lack FTM focussing and have a fairly poor record reliability-wise.

Micro USMs:

How can you tell which type is fitted to which lens? Well, as you might expect Canon deliberately do not distinguish between ring and micro-USM in the name so you have to look up the specs for the individual lens to find out. Ring USM is definitely worth having but if it comes down to a choice between micro-USM and ordinary micromotor focusing, I'd save my money and go for the non-USM version

Finally, just because a lens has USM doesn't mean it has any better optical quality - it's just an AF mechanism

Steve H
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