I own a nikon d200 and im looking for a fixed f2.8 off brand lens that is under $600 with a zoom range of at least 18mm to anywhere above 35mm. A few that I have found so far are the Sigma 18-50mm f/2.8 EX DC Lens and the Tamron 17-50mm f/2.8 XR Di II Lens. Are there any other lenses that offer a greater zoom range & cost <$600? If not, then which of the two would you guys recommend? I have read some reviews about the Tamron lens being slow at focusing under low light conditions but other than that, the tamron wins over the Sigma in every other aspect. What do you guys think? Anyone here who actually owns one of these lenses?