I've got the Nikon D50, Nikon 18-55, and a Tamron 70-300mm macro, AND I'm on the hunt for another lens that will allow me to get closer to birds, as well as any other subject. But, I'm not sure why I'd need it to be "faster", as some seem to say about one lens or another. For instance, I read in a thread where the poster said that the Sigma 50-500mm lens was much faster than the 170-500mm Sigma lens. I'm a complete rookie, so don't laugh. Is it just that the lens Auto Focuses faster, or what? I'm buying a lens to get me closer, that I know, but what will the lens do IF it is one of these "faster" lenses ALSO.
My kids are active, so might use this lens to shoot some sporting action type photos as well, if that helps!
Thanks again for the schooling...
Kevin