I have a D80 and a D90. Butterflies were shot at the Wild Animal Park. During two weeks in April there is a special butterfly exhibit. I used a 70-300 with an extension tube and external flash. The Tokina 11-16 f/2.8 has very good reviews and it is a good alternative to the new and expensive 10-24 Nikon DX. You will need a D80 or higher for the Tokina if you want the camera to auto-focus.
