>>4172391Missed this reply. Sorry.
A fast lens could certainly help you when shooting conventions, and it makes sense to stick to zooms. But, for your purpose, I wouldn't recommend the Sigma 18-35mm. The range is pretty limited and it's not made for portraits. It's a bit too wide for good bokeh. Also, portraits shot below 35mm are usually unflattering.
I think a 24-70mm f2.8 (or similar) would be a better match for you. It's pretty wide, so you can still capture crowds and general convention pictures if you feel like it, but it also has a great portrait range (35-70mm) so you can do full-body and medium shots. The 18-35mm may be fast, but it's too wide for nice portrait bokeh. Good bokeh can be essential in an environment with lackluster lighting, as can help you separate the subject from the background (which can be pretty darn cluttered at conventions). f2.8 is still really fast, so don't worry about the speed.
If you decide to go with a 24-70mm f2.8 (or 28-75mm f2.8), here are some tips: Canon's own lens is pretty darn soft, so you may want to go for Sigma or Tamron's offerings. Tamron has three options: their G1, G2 and a 28-75mm. The G1 and their 27-75mm can usually be found used for around $350-400 (same as their 27-75mm). They're both excellent budget options, but can no longer be bought new (don't ever buy new). The G2, which is slightly sharper at wider apertures, has nicer build quality and better stabilization, can be found used for about $700 ($1200 new). Sigma usually sells for a bit more ($1300 new), but has no discernible benefits over the G2.
I hope this helps.
