The shots I was disappointed with yesterday were at 4.9, 1/400 to 1/800, non-IS, ISO 200. That's something the lens alone could typically handle, so maybe I'm underestimating the added focal length. When I stopped down to 5.6, images appeared sharper, but it was a different subject/lighting, so it's a difficult comparison. I've tested the TC on the camera and lens and found no discernible image degradation, but once I get in the field I see it time and time again. I should probably broaden testing to include different light conditions and IS.