12-06-2012 02:24 AM
I think you're getting a little confused here. STM is a type of focus system, like the ring USM in L lenses. The II (2) just denotes that it's the second iteration of a certain lens. For example, 24-70 f/2.8L and 24-70 f/2.8 L II. Knowing whether or not the STM focus is better than whatever other lens you're looking at would first require us to know what it's being compared to.
08-21-2013 10:06 AM
I don't have the STM lens but from what my movie making friends tell me, it is faster at focusing and quieter. Both of which make better movies. I guess still photographers would benefit from faster focusing too. If I had the IS II version I would not go out and buy the STM version. But if you didn't already have it, than sure, it's probably the way to go.
08-21-2013 11:37 AM
Note that with STM the focus ring is electronic. This means that if you use manual focus you turn the ring to activate a motor that then seeks focus, instead of a direct mechanical link like in a traditional lens. There’s a bit of a delay. I think it’d bother me, but others might like it.
I’ve read good reviews on the 18-135 STM, but the primes don’t seem like the AF speed is anything amazing.
08-21-2013 03:37 PM
You can watch (and hear) the difference. Here's a YouTube video I stumbled across: http://www.youtube.com/watch?v=tm8C76c-jtA
The 18-135mm STM is quiet to the point that I cannot hear a thing. The 40mm makes a _very_ slight hum. The 18-135 non-STM makes quite a racket and is very slow.
Unfortunately the person recording the video to compare the focus speed of a lens with USM motors. Those lenses tend to be QUITE snappy.
The USM lenses allow for full-time manual focus even when in auto-focus mode because of how the system works. You can think of the focus ring as mechanically engaging the focus... but with a "clutch" adjusted to allow slippage. The regular lenses (non-USM and non-STM) use gears -- there is no slippage and Canon recommends against attempting to focus the lens manually if it's in "AF" mode. The STM lenses allow for full-time (well... semi-full-time) manual focus with the caveat that there is actually no mechanical linkage to the focus system at all... it's completely focus-by-wire. So when you turn the focus (manually) you're really just sending commands to the computer to move the focus mechanism. This means the camera has to be "awake" to operate the lens (whereas with other lenses you can even focus the lens when the camera is completely powered off.)