I would consider a 135 F2.0L, which gives you a great distance lens so you don't have to be in their face when shooting, or like someone mentioned an 85mm. This depends on the camera you are shooting with. I don't recall if you mentioned that, but if you are shooting on a crop and not a FF camera, the 135 might be too long. I think the probably best option you have is the 24-70 F2.8L or version two of the same lens. I know you said, max 2.0 in aperture, but this lens gives you a lot of versatility and is often used for this specific purpose. If you were shooting below stage, I would opt for a 70-200 F2.8L IS II USM on a FF body. 50mm would limit me, but if on a crop body, it is a great focal lenght.i w lenses.
has no one tried the sigma 17-50mm 2.8 lens before?
I got the sigma 17 - 50 about a week ago. My other lenses are canon so I was a bit nervous. There seems to have Quality control issues in the past as in the first year or so that these came out. Many seemed to have dead on accurate auto focus yet others seemed to be slightly off needing calibration.
As far as I can tell the chances of receiving a good copy has improved overtime, at least it looks like that from the reviews that I've been reading.
My copy seems to have accurate auto focus, very nice build quality, quiet focusing mechanism and very good optic quality. I have actually used both lenses side by side. The Canon and the sigma versions have similar handeling.
In my opinion the sigma feels slightly more durable higher quality overall in your hands.
Canon has the nice full time manuel/autofocus though. Haven't decided for sure but I think canons image stablization is just a little better then the sigmas.
Canon provides optical in camera correction with many of OEM lenses including the 17-50, though the sigmas optics as far as I can tell are quite good. Thus far j pegs straight from the camera are very good not requiring any particularly crucial post processing.
Well that's my experience so far.
Oh right I'm using a 60D body.