Quote Originally Posted by Jon Ruyle
Yet the 200 f/2 has a basically perfect in-focus image wide open.

Maybe I'm wrong. My information came from an astrophotographer on Cloudy Nights; he said that the 500mm f/4 IS and 600mm f/4 IS were well corrected (and therefore highly suited to astrophotography), while the 200mm f/2 was undercorrected for bokeh.