lycan said:
Soft at 600mm? I don't think so
I've seen plenty of full-sized shots at 600mm to know that it is soft at 600mm wide-open, even on full-frame.
I'm not a sharpness nut, except on telephoto lenses, where it's common to crop like crazy. When I can properly frame, I find my 17-40L and 24-105L to be quite excellent. But on telephoto, it's not uncommon for me to crop 2x into a shot that was shot with a 2xTC on a 1.6-crop camera. That's only 15% of the size of the image circle, enlarged to a full-frame. That requires critical resolving power.
Have a look. This is the Tamron against itself (400mm versus 600mm). There's a substantial difference.
http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=929&Camera=453&Sample=0&FLI=3&API=0&LensComp=929&CameraComp=453&SampleComp=0&FLIComp=5&APIComp=0