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My Canon lenses have the designation USM on them, which I assume is for the same reason that they say "Ultrasonic". What does this mean, and why or why do I not want it?

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5  
This is probably covered at a high level by What do all those cryptic number and letter codes in a lens name mean? –  chills42 Nov 4 '11 at 15:16
    
I guess we should probably close this as a duplicate, since the wiki chills linked is pretty much the source for such information. Any objections? –  jrista Nov 5 '11 at 2:55
2  
@jrista: My only argument there is that I think that the last phrase (why or why do I not want it) makes this a different (more specialized) question. –  chills42 Nov 7 '11 at 16:09
    
@chills42: Good point. I've left it open anyway, so I guess all is good. ;) –  jrista Nov 8 '11 at 6:40
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With that in mind I suggest we mod the title to "What is USM and what are the Pro and Cons" or similar –  Gapton Nov 8 '11 at 14:52

4 Answers 4

up vote 25 down vote accepted

USM - Ultrasonic motor (This is the Canon Terminology)

This is a big improvement over older micro-motor based autofocus systems, which are significantly slower and louder. There are two types of USM systems "Micro" and "Ring". The preferred type is "Ring Type" which always allows for manual focus without turning off auto-focus. Most, but not all, Micro USM lenses from Canon also have full time manual focusing.


Benefits of Ultrasonic motors:

  • Faster focusing
  • Quieter
  • Full time manual focus (for ring-type USM and many but not all Micro USM lenses)

Downsides:

  • Higher Cost

Branding

USM is a Canon trademark, so similar terms are used by other manufacturers. These other names include:

  • USM: Ultrasonic Motor (Canon)
  • SWM: Silent Wave Motor (Nikon)
  • SWD: Supersonic Wave Drive Motor (Olympus)
  • SDM: Supersonic Drive Motor (Pentax)
  • SSM: In-Lens Super-sonic Motor (Sony/Minolta)
  • HSM: Hyper-Sonic Motor (Sigma)
  • USD: Ultrasonic Silent Drive (Tamron)
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another downside: weight. Another name for USM Nikon uses is AF-S (Auto Focus - Silent wave). –  jwenting Nov 10 '11 at 10:26
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"...The preferred type is "Ring Type" which always allows for manual focus without turning off auto-focus." Can you provide an example of a Canon lens that includes a micro-USM motor that does not have full time manual focus? I've not come across one that doesn't. The EF 50mm f/1.4 has a micro-USM motor and certainly has full time manual focus. –  Michael Clark May 7 at 23:19
    
@MichaelClark: EF 75-300mm USM (non-IS) series has micro-USM but not full-time manual focus. –  crunch Jul 14 at 16:23
    
The EF 75-300mm f/4-5.6 USM III is not a lens, it is a paperweight pretending to be a lens. :-) –  Michael Clark Jul 15 at 0:00
    
OK, so there is at least one Micro USM lens that does not have FTM focus, but the majority of Micro USM lenses do and that should probably be reflected in your answer. –  Michael Clark Jul 15 at 0:02

Ultrasonic Motor

You want it for a few reason:

  • nearly silent AF (important for photographers working with animals)
  • Full-time (even when AF is on) manual focus (important for macro)
  • It is much faster than non-USM focusing (especially useful for tracking moving subjects)

You may NOT want it because:

  • Lenses with USM are more expensive

Call me dumb but after few minutes of hard-thinking I still cannot come up with ANY other reason you may NOT want it. Making my answer almost identical to the previous one lol.

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Ultrasonic motor (USM) in this case should not be associated with human being audible frequency. I do not think the lens manufacturer builts it for the reason of no noise during operation, but the advanced simpler control of the lens movement by using the sinusoidal electric wave at the ultrasonic range frequency. Ultrasonic motor is low speed and high torque, and other than any of electric motors with high speed and low torque. Accordingly, such of focus hunting can be eliminated by the characteristic of USM and eventually we can obtain faster auto focus with.

Ultrasonic motor is anyway more expensive to be compared with any of electric motors.

The preferable kind of USM is ring type, which involving few rotating/moving parts and so the simple construction, so that resulting very fast performing autofocus, nearly no noise and we can rotate the manual focus ring at any time regardless the focus mode. Another construction is micro USM, which is working as well as conventional electric motor. Even so, the advance of low speed and high torque remains benefit for fast auto focus. As well as the conventional motor, some sprockets are required for connecting the motor and the frame of lens which therefore more mechanical constructions is required rather than the ring type. Consequently, we can hear a harder noise during the operation and little slower auto focus. The auto focus, still, much faster comparing to other type of electric motors.

As well as the system construction is nearly same with electric motor, the manual focus ring will be rotated as well when it is set under auto focus mode. The only switching to manual focus mode allows us to rotate the ring manually. The exception is EF 50 f/4 (micro) USM. You can rotate the focus ring manually at any time regardless the focus mode. A complex sprockets arrangement (look alike the latest car with automatic transmission, the wheel can be rotated at any direction under the engaged gear without damaging the gear box) leads us to operate the lens as well as by using the ring type.

I wish I can contribute to the discussion.

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As many people have mentioned USM = Ultrasonic motor. Its a small motor built into the lens. This means that the camera will focus MUCH faster without making noise (or much noise). There are actually 2 kinds of USM, "Ring Ultrasonic" (good) and "Micromotor Ultrasonic" (not so good) see here. If you're interested in a more technical explanation form canon, see here.

Amazon shows a 50mm lens at $100 for the non USM and $300 for the USM. AFAIK its the same glass in in both, so you're paying for the quick and quite motor. Its also heavier.

Pros

  • Fast and Quiet AF
  • Manual focus in AF mode

Cons

  • Heavier than non USM lens
  • Significantly more costly
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2  
Which two 50mm lenses are you comparing? I don't think they actually are the same glass.... –  mattdm Dec 28 '13 at 3:24
    
I'm comparing Canon EF 50mm f/1.8 II Standard AutoFocus Lens AND Canon EF 50mm f/1.4 USM Standard & Medium Telephoto Lens for Canon SLR Cameras –  sevensevens Dec 28 '13 at 17:05
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Those lenses are actually different internally. The wider aperture is one clue, of course, but the f/1.4 also has an additional lens element (7 elements in 6 groups vs. 6 elements in 5 groups). Canon also says the f/1.4 uses high-refraction glass for two of the elements. And it has a larger filter thread and is larger in diameter overall. Also, the f/1.8 has a plastic rather than metal mount, which shaves a bit from the price. I think your basic pros and cons are still right, but USM is only a fraction of the difference in both weight and cost. –  mattdm Dec 28 '13 at 18:07
    
I'm actually in the market for a 50mm (I'm still researching everything). I still in the amateur area, but wouldn't mind paying a little more for a USM. Does Canon have a 50mm prime lens with USM that is cheaper (I have a Rebel t2i btw). –  sevensevens Dec 28 '13 at 18:45
    
No, just that. Honestly, I wouldn't make it a deciding factor, because it is the less-good ultrasonic micromotor. You might look at the 40mm f/2.8 STM, though (nicely reviewed at dpreview; the second page compares STM vs. USM autofocus). You might also investigate the Sigma 50mm f/1.4, which uses a ring-type ultrasonic motor. (But isn't cheaper.) –  mattdm Dec 28 '13 at 19:03

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