# Difference between DC and USM AF?



## inTempus (Dec 27, 2008)

I'm looking at buying a Canon EF 75-300 and the price on Buy.com seems pretty good.  In the description it says that it uses a DC drive for the auto-focus vs. a USM.  What does that mean and is that bad?

Thanks!


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## rufus5150 (Dec 27, 2008)

In the Canon world, USM are quiet and fast. Non-USM are not (in my experience).


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## Jaszek (Dec 27, 2008)

USM stands for Ultra Sonic Motor, which means that it's silet b/c there are magnets in the lens instead of a motor to focus. And I have no idea what DC is lol. I gues it means not USM.

_YAY 100th post _


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## gsgary (Dec 27, 2008)

Have you looked at the Sigma 100-300F4 much better lenses


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## inTempus (Dec 27, 2008)

Thanks everyone.

I haven't looked at Sigma lenses.  I was reading on one of the review sites that their lenses are hit or miss on how high quality their optics are.  One reviewer said he got the same lens 3 times (original purchase + 2 replacements) and the each had their own problems.

Was that just a fluk?


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## Josh66 (Dec 27, 2008)

I don't think flukes happen three times in a row...

I would stick with Canon glass, but if you want to go 3rd party - that's fine too; lots of people seem to like them.  (Then again, you hear stories like the one you heard...)

I have the lens you're looking at (the Canon 75-300).  It's OK, especially for the price.  With lots of light you can still get pretty good results from it.  Just don't be expecting "L" quality.


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## Josh66 (Dec 27, 2008)

Back to the main question though - the difference.  The most important difference for me is that non-USM lenses don't have full time manual focus (you have to switch it over to manual, on a USM lens you can manual focus even when the lens is on AF).

USM is a lot faster too, but that's been said already...


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## tsaraleksi (Dec 27, 2008)

USM also tends to be more accurate.


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## gsgary (Dec 27, 2008)

tharmsen said:


> Thanks everyone.
> 
> I haven't looked at Sigma lenses. I was reading on one of the review sites that their lenses are hit or miss on how high quality their optics are. One reviewer said he got the same lens 3 times (original purchase + 2 replacements) and the each had their own problems.
> 
> Was that just a fluk?


 

The 100-300 is a top lens FM Reviews - APO 100-300mm f4 EX IF HSM


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