guys i would like to know. i tried finding here about comparing these 2 lenses which one gives u a better quality pics and also the performance.. all 2 cameras do have low F stops.. but is there a difference between tamron, sigma and nikon lenses in terms of the quality and the performance?? i read like sigma its hard to get focus on u know sumthing lyke that.. hope i can get some advice from you guys.. thnx.