I'd be curious about opinions on which Nikon telephoto lenses you prefer for low light situations, at the edges of the day or on heavily overcast days. I currently shoot with a 500mm f5.6 lens and, while generally I get OK results, AF really struggles in the early morning and late evening hours when light is low, and images end up at very high ISO, thus compromising the quality of the images. I'm considering acquiring a faster lens - f4 or wider - in an effort to obtain better results. For those of you who have shot in low to very low light situations with such lenses, what has been your experience?