Let's imagine you can get close enough to the croc with your D700 and 300mm lens for it to cover 1000 pixels horizontally in the frame.

At that same distance, replacing the 300mm on the D700 with a 500mm would allow the same croc to cover about 1670 pixels. On the other hand, by keeping the 300mm and replacing the D700 with a D800, the croc would cover 1730 pixels, so purely in terms of number of pixels you'd be slightly better off getting a D800 than a 500mm.

Brian
Welsh Nikonian
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