profpb wrote:
MT got it right, but what's new? I have the 28-300mm and when I want 450mm I choose between putting it on my D7000 or my D800 in DX mode.
I have outgrown teleconverter-envy by simply cropping my 36MP images. No loss of light, no damaged lenses, and more money for my future purchase of the 600mm when my wife says its o.k.
Your second paragraph is the way to go. With the resolution of the 800 and the quality of the 28-300, it allows us to crop and get the benefit of a much longer lens (I use the same setup). Using the lens in DX mode, however, just means doing a crop in camera. It's still a 300mm lens at most.