The serious lacking of range on full frame camera lead me to this lens (after coming back from Yellowstone), considering the cost, size and weight, this is perfect for me. It's not as sharp and as contrasty as refractive optics, but it still captures lots of detail. The key thing about using this lens is to keep the shot steady, and use high ISO to achieve high enough shutter speed. Another interesting thing to know is Nikon has an earlier version of 500mm mirror lens, which is 500mm F5 (3kg and 38.4cm long) in non AI mount- it's also very rare. This one is just 840 g and 10.9 cm long.