Many of you helped me out last week when I was looking to upgrade from my Sigma 150-500, thank you. Now I would like to move away from the Tamron 17-50 VC lens I have because I've noticed he auto focusing is a little on the slow side and jerky when it engages. I'm good with spending up to $850 but found it really confusing looking at different reviews. Some like the Canon 17-55 but others said IQ is no different from the 15-85 which offers more range for less money. I even saw a fairly good review for the Sigma 17-85. I'm turning to the experts once again, what should I do?