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Canon USM

pqbon and I have been trading lists of lenses that we are lusting after, and I've been realizing that all this Canon lens terminology has been leaking from my brain faster than I can put it in. I've decided that I'll do a series of posts representing various things I've learned on the Web about Canon equipment, as well as photography in general, in the hopes that I can slow the rate of leakage.

One bit of Canon terminology that is troubling is USM, a technology for autofocusing that stands for "Ultrasonic Motor." USM was a term they first started using in 1987 when they developed a new fast and quiet motor, which they dubbed a "ring ultrasonic motor" -- the motor uses tiny vibrations to generate rotation. As the term "ring" implies, the autofocusing mechanism sits in a ring around the optics of the camera, and most importantly, it allows you to do manual focusing even when in auto focus mode. Steve Weixel put together a nicely photographed dissection of his broken ring ultrasonic motor. Here's a photo of the ring motor itself:

04-19-05.ring-motorb.jpg

Several years later, Canon developed a new autofocusing technology that is a motor that connects to the focusing mechanism via a set of gears. This mechanism is cheaper to produce, and Canon also touts the ability to stick it into smaller lenses. As you can see from the photo below, this new motor is considerably different from the ring-style motor:

04-19-05.ultraimage2.jpg

The downside of this new motor is that it is louder than the ring-style mechanism and in most lenses that it is used in you cannot do manual focusing when in auto focus mode -- you need to check if the lens has "FTM" (full time manual) as a feature.

Blessed with creativity, Canon decide to call this new technology "micro USM." In an attempt to conserve letters, Canon shortens this to "USM" in its product names and instead of having a red-rimmed ring around the front of the lens they use a golden ring (update: red-rim indicates "L" luxury lens, which happens to generally be an indicator of ring-type USM, but there are several ring-type USM lenses that are golden-ringed -- see comments).

If you read this photo.net thread, you'll find people listing all sorts of other 'tests' you can do to see what type of USM motor a lens has, though the easiest method is probably just to use Google to find out which type your lens has.

Does the difference between the two lenses really matter? Depends on you, really. Micro-USM saves you money, and I'm also so used to flipping a switch between autofocus and manual focus. Another caveat is that you have to be careful that you are not in sports mode on a Digital Rebel, as in sports mode the auto-focus mechanism will try to continually autofocus in order to follow a moving target (AI Servo mode). So, for me it probably doesn't matter, but I do enjoy the feel of the ring ultrasonic motor on my 70-200mm lens.

Link summary: * Canon Web site on Ultrasonic technology * Philip Greenspun posting on photo.net about Canon EOS lens * Dissection of an ring ultrasonic motor

Comments (3)

Ken,

IIRC the red ring indicates an L lens not ring USM. The EF 28-135mm f/3.5-5.6 IS USM has no red ring but is ring USM...
http://consumer.usa.canon.com/ir/controller?act=ModelDetailAct&fcategoryid=149&modelid=7337
The EF 70-300mm f/4.5-5.6 DO IS USM is also ring USM with out the red ring.
http://consumer.usa.canon.com/ir/controller?act=ModelDetailAct&fcategoryid=150&modelid=9996

I couldn't find an L lens that didn't have ring USM. but that isn't really a suprise.

Herman:

Hi, pqbon,

Do you know how to drive the micro USM? Actually, I would like to know how to control the movement of micro USM via those 4 wires mounted on it.

Best Regards,
Herman

Cem Günay:

Can you use a usm motor to auto focus in low light conditions to dark conditions because the standard motor on my canon lens needs light to auto focus. Im wondering if you can use a usm lens in clubs or concerts for profesional photography.

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