In scanning the many responses concerning 400-500mm telephoto lenses when used in nature photography, I only seem to see the Sigma 400mm mentioned more than any other 3rd party lens. I recently acquired a Tokina 400mm f5.6 SD ATX (for Nikon) and, after reading the many comments, am beginning to feel that I may have made a mistake. Do I have something that has passed it's prime? Where does this particular lens rate as compared to the "apparently much more popular" Sigma version of the same kind? I still have time to change before a majoe Yellowstone venture in a few months. Thanks much.