STM vs USM

So with the STM lenses coming out to aid auto focus during video and the new 70D presumably going to have this feature, is there any reason to get one when I have a 17-55 and the 70-300L? Does the STM offer something that ring USM doesn't do for autofocus during video?

Just another variation.
STM is Stepper Motor and AF is still AF with its flaws.
Wait for many user evaluations but prob little significant difference.
Better in the noise department for video folks.
http://photo.stackexch​ange.com …-stm-mean-on-a-canon-lens

If you're rack focusing in video using manual focus, then the 40mm pancake's STM mechanism is actually worse because you'll hear the fly-by-wire motor if you use the onboard microphone. The AF video on the t4i and t5i that I've seen with the pancake seems pretty good, albeit a little slow to grab focus at times but its definitely quiet enough if you're in any venue that isn't silent. Compared to USM its definitely quieter for video AF for sure, just not 100% silent.

Keyan wrote in post #16076166
So with the STM lenses coming out to aid auto focus during video and the new 70D presumably going to have this feature, is there any reason to get one when I have a 17-55 and the 70-300L? Does the STM offer something that ring USM doesn't do for autofocus during video?

Yea there are a few reasons that may be good for you.
#1 STM can change it focus speed for video, USM cant do this. The advantage is in follow focus the USM will have fast little jumps and the STM will be one smooth movement.

#2 the STM on the 18-135 stm is VERY silent, you have to have your ear next to the lens to hear it.

#3 the 18-135 STM is the only lens in the canon EFs or EF lens line that has dynamic IS for video, its like using a steady cam.

But on the other hand if your not doing video on crop its no use but for the silent focus on zoom STM's.

Best of luck

Its a trip for me to see stepper motors in consumer products. Dad used to sell stepper motors to industrial clients. I grew up with them, took demo's into career and technology fairs in grade school. Still remember the sound from the dining room as he'd program a demo - the whine noise of different speeds (most were larger than a lens). STMs work by way of counting steps, each a fraction of a degree around the circumference of the motor. Instructions from the software go something like:

rotate clockwise for x steps at y speed
rotate counterclockwise for x steps at y speed
rotate clockwise for x steps at y speed
rotate clockwise for x steps at y speed

Starting is precise, movement is precise, stopping is precise. And it turns out that for lenses, stopping is the most important (and difficult) part of autofocus. Older lenses push in a given direction but sort of guess at where to stop. Turn off power and it will probably land where its needed. Slower focus lenses actually had an advantage because stopping distance is easier to calculate.

In this age of precision, this is no longer enough. There are two ways to get hyper focus with a motor:

USM motor with focus detection unit, to measure the landing spot
Stepper motor with instructions to stop at the perfection spot

The best focusing newer lenses have one or the other. This doesn't really help video for those cameras without video autofocus, but either is a major improvement for still shots.

Varago wrote in post #16076554
Yea there are a few reasons that may be good for you.
#1 STM can change it focus speed for video, USM cant do this. The advantage is in follow focus the USM will have fast little jumps and the STM will be one smooth movement.

#2 the STM on the 18-135 stm is VERY silent, you have to have your ear next to the lens to hear it.

#3 the 18-135 STM is the only lens in the canon EFs or EF lens line that has dynamic IS for video, its like using a steady cam.

But on the other hand if your not doing video on crop its no use but for the silent focus on zoom STM's.

Best of luck

I have a 18-135mm STM and I agree,, the auto focus is dead quiet. On my 50D and 20D it's also very fast.

I bought this lens for shooting videos on my EOS M via the adapter. In this situation, the 18-135mm STM you can't hear a thing as it focuses and is reasonably quick.

For a kit lens, the 18-135mm STM is very sharp. Great value for money.