hi to all, i have a tamron 17-50mm f2.8 nikon mount... i'm using it for day shots at the most, in a few ocassions though, i have to shoot using the tammy in a low light situation. i noticed that the AF is damn slow... sometimes, i cant even get the shots i want because by the moment it can AF, the subject has gone, or blocked, etc... so, i have to turn the AF off to make a fast enough shot. newbie question here... is it a normal behaviour of the tamron 17-50? or is there somethin wrong with my lens? educate me please... thanks!