Ideally I would like to buy the Canon 17-55mm 2.8, but at around $1000 this is too much, at least for now. Please advise on the best alternative from the following 3. For 7D body.
a) Tamron AF 17-50mm F/2.8 SP XR Di II VC - $549.00
b) Sigma 17-50mm f/2.8 EX DC OS HSM - $669.00
c) Enter your own suggestion if you think there is another standard zoom, constant aperture lens which I should consider