The EF-S 17-55mm f/2.8 IS USM offers an f/2.8 focal ratio available at all focal lengths in the zoom range. This boosts performance in lower light and also allows for a narrower depth of field (when you want to deliberately blur the background but have a tack-sharp subject -- although that works better at longer focal lengths because depth of field naturally gets broader at short focal lengths.) USM lenses have very fast focusing motors. USM lenses allow for continuous manual focus at all times -- even when the camera has auto-focus swtiched on. This is because the focus ring has a clutch which allows the ring to slip (intentionally) against the focusing mechanism -- so you can't hurt it if the camera wants to go one way and you want focus to go the other way. Lenses which regaular auto-focus motors should not be manually focused unless you switch off the AF system to avoid damaging the mechanism.
The STM lenses are faster to focus than the standard kit lenses. They're not quite as fast as USM but much faster than standard focusing motors. The focus performance is great for video. The STM is also designed to be extremely quiet -- so quiet that if you record video using the internal microphone the camera will not be able to pick up noise from the focus motors (sometimes it can... but it's very faint.)
STM lenses are "focus by wire". There is no mechanical connection between the focus ring on the lens barrel and the actual focus mechanism inside. Instead it senses that you are turning the focus ring and the motors move focus with you. This means you get "full time" manual focus even when the lens is in AF mode... but the lens has to be "awake" for this to work. If you release the shutter button... after a few seconds the camera's metering and focus system goes to sleep (but the camera stays on) -- this saves power of course -- but it also means that turning the focus ring does nothing. You have to touch any button that wakes up the focus or metering system (e.g. half-press the shutter will do this) and then the STM lens will follow your focus changes.
The EF-S 18-135mm f/3.5-5.6 IS STM is going to be a bit more versatile than the 17-55... given it extends the focal length of the lens by another 70mm. This makes it a nice all-around general purpose lens without going overboard.
I stay away from the "super zoom" lenses... these are lenses that offer a VERY broad zoom range from short to long. E.g. an 18-270mm is a "super zoom". These lenses are convenient but that's the only thing they have going for them. It's difficult to make a lens with a very ambitious zoom range AND have very good optics at the same time. Lenses are often a game of trade-offs... part of the reason your camera allows you to swap lenses is because no single lens is ever the "best" at everything. Buying a super-zoom where one lens "does it all" is a bit counter to the whole point of having a removeable lens camera. You end up with a single lens which tries to be "jack of all trade ... master of none".
Zoom lenses with less ambitious zoom ranges tend to have better optics. e.g. if you divide the high end value of the range by the low end value of the range (e.g. 55 ÷ 18) and it divides in some 3-5 times... that's great! If it divides in less then 10 times (e.g. 135 ÷ 18 = 7.5), that's good. If it divides more than 10 times then the lens is a "super zoom" (e.g. 270 ÷ 18 = 15) and the optical quality starts to get mushy on these things.
With that said, times are changing and optics are getting better. Todays entry-level lenses are pretty good compared to even some fairly expensive glass of past years. Once a lens is designed it will likely sell for years and years without an update. Lenses designed within the past few years tend to be a bit better than the lenses of just a decade ago (assuming the lenses are in the same class.)
I left out Canon's "L" series lenses... these are Canon's highest end lenses. But there is a rule that to qualify as an "L" series lens, in addition to having higher end build, optics, and other areas of performance, the L lenses also have to work on EVERY Canon EOS body, film or digital, past or present (and presumably future). Since EF-S lenses cannot work on film or full-frame bodies, they don't qualify as "L" series regardless of their quality.
Tim Campbell
5D III, 5D IV, 60Da
