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Works well wide open. As it should.
D800, f/4, 1/160, 500 mm, ISO 1600
momma squirrel by Christian Hunold, on Flickr
Nearly wide open - f/4.5 seems to offer slightly more vivid contrast.
D800, f/4.5, 1/1000, 500 mm, ISO 280
great blue heron by Christian Hunold, on Flickr
Stopping down to f/5.6 gets you a little more depth-of-field. I don't have much experience with smaller f-stops; the odd shot taken at f/8 has come out looking okay.
D7100, f/5.6, 1/1000, 500 mm, ISO 1100
Whitefoot by Christian Hunold, on Flickr