Hi, I'm looking to fill a gap in my lens collection and want to get a zoom lens for a Canon T4i that has a fairly low f/stop (2 or 2.8). I currently have the Tamron 17-50 f/2.8, and a Canon 100mm IS, also a f/2.8. And the Tamron 18-270 (for walk around use).

I'd like to see what my options are for something reasonable (less than $750 if possible) to fill the gap from 50-maybe 105 at a low f/stop. I've seen the Canon 24-105 but thats a fixed 4.0 on the aperture and I'm just not sure about that one. Any help, suggestions, pointers or tips would be greatly appreciated!

Thanks in advance!