Tamron 150-600mm f/5~6.3 is a popular choice for a truly LONG zoom lens. Sigma has two 150-600mm models, both new, like the Tamron, and the lighter SIgma version, the "Contemporary" model is priced almost the same as the Tamron 150-600, and is priced very close as well. The Sigma Sport model weighs about two pounds more than the other two lenses, and costs another nine hundred dollars or so, but seems from what I have seen on-line, to be the best of the three lenses--buuuuuut, it's in the 6-pound lens class, which makes for a very front-heavy balance on a small,light camera like a D5100.