The Nikon 500mm primes are pro lenses, and the price tag, even used, reflects it. But with the crop sensor and teleconverter it sounds like your reach is about 1050 mm, and that is very interesting. No wonder we can see the details in the owls eyes and feathers. With my Nikon 200mm f/4, 1.4 teleconverter, and in 1.5 crop sensor mode, reach is 420mm max, and that is too short. The Sigma Sport is probably a bit sharper than the Nikon 200-500, with better reach, but it is a beast. If I pulled my 200mm out of my bag on occasion, and replaced it with the new Tamron, or Sigma C, or a used Nikon 500mm, and went to the beach, or Forsythe. I would have the reach, though perhaps not Nat Geo quality.