I'd like to get a longer lens and while I'd love to get a super fast f2.8, can't justify the expense. I was tossing up between the Nikon 300mm f4 or the Sigma 150-500 to use on my D7000. I think from reading reviews the 300mm would still AF with a teleconverter on. A friend uses the Sigma on her Canon 7D with really good results and even hand holds a lot of shots. Any input from members who have either would be appreciated.