I have a Nikon D90 and my longest current lens is the Nikon 70-300 mm f4/5.6 AF-S VR IF ED. I would like to increase my focal length range in order to improve my bird and wildlife photography, and am considering purchasing the Sigma APO 150-500 mm f5/6.3 DG OS HSM telezoom. Although I know the value of working with prime teles when it comes to birds, I simply cannot justify purchasing a $5000 - $10,000+ lens. If anyone has had experience in the field working with this lens, I would very much appreciate your input. Many thanks!