So i'ts for Full frame, the 5D III
Can you tell me the overall image quality of the Sigma 85mm f1.4 vs Canon 135mm f2.0
(resolution, CAs, focusing..etc)
I own the 135 L and recently rented the Sigma 85 for a week. Based on that somewhat limited experience I would say that image quality is very similar; for me, the main difference is that, thanks to the longer focal length (presumably), it's even easier to get really smooth backgrounds from the 135L than the Sigma 85 (the same's true of my 200 f/2.8 L, but I've no idea how well that works on human faces...). Both are extremely sharp (perhaps the 135L is a bit sharper wide open than the 85, but you really have to look for the difference), and I've not noticed any CAs with either one. The135L focuses amazingly fast, considerably faster than the Sigma (the copy I rented, anyway) but the Sigma seemed just as accurate when it got there (and it wasn't exactly slow anyway). I've no idea how either one compares to the 100mm f/2. The Sigma produces somewhat smoother backgrounds than my Canon f/1.8, but again doesn't focus anywhere near as fast. (I use a 5DII.)
However, since you have a 70-200L f/2.8 you might first want to ask yourself (1) what it is that you want from a lens that the 70-200 can't provide, (2) which focal length you find most useful/desirable out of the three, and (3) if you're photographing weddings, is a zoom more useful anyway? You may not need to buy any of these ....