Everyone gives this lens a rave so maybe i should approach this terrific lens from a shooting POV.
Shooting wide open at f2at 135 gives your pictures a distinct quality you would never get from zooms, there isn't a zoom at this focal length that is this speed either. A few primes come close but they're nowhere near as sharp wide open.
the af is fast and very accurate and the lens has a great feel on a 20d/5d/10d body even wo the battery grip of the cameras.
Close up magnification of .19x to.21 therabouts is a nice bonus of this lens, letting you get closer wo being too in your face for tight headshots.
I always felt that photographers usually have a sweet spot when shooting certain things like some portrait photographers like the distance the 85mm gives them when approaching a subject, turns out I'm liking the 135 focal length quite a bit for headshots and the 85 for torso and head shots.
One thing I like about this 135/f2 combo is that there's enough DOF to get most of the main subject in view and focus but the background will still be blurred out. It's really great when you want the subject in focus and nothing else, as opposed to the 85 1.2 parts of the subject could melt into the background resulting in images that be a little too ethreal for some tastes. Couldn't you just stop down the 85? Sure but the relationship of the foreground to background would still be different with the 135.
The 135 f2 can give a result that's juuust right. The backgrounds just a little more compressed with the foreground but the foreground is sharp but the background is gone. Almost like you lurred the background in photoshop. But you'll never get the same results from blurring in photoshop, for one thing the highlights would get muddied in a blur treatment in PS while the highlights stay clean at f2.
Don't just use this thing for portraits, use it the way you would use a 50mm and try and frame as such, again the focal lenth and wide aperture will give an interesting result. I basically use it for candids and portraits during a wedding.
Fantastic lens, very sharp, very handholdable, the focal length is terrific for portraits and the bokeh is very beautiful, equal in sharpness to the 85 1.2 or any of Canon's $5k lenses. This is probably the best lens you can get for the price, the only lens I've used with better saturation and contrast is the 35 1.4L.
Downside: Not as handholdable as the 50,35 1.4 or the 85 1.2 during low light situations due to it's focal length.