Got a Canon 400mm f/5.6L from UPS today. I still have to take it out in the field, but despite the lack of image stabilization it’s possibly the sharpest lens I’ve tried:
This is handheld with flash but no tripod and wide open!
The lens is also surprisingly light. However the minimum focus distance is quite long: 3.5m.
The 300mm + 1.4x teleconverter didn’t do nearly as good a job, even on a tripod:
The 300mm with no teleconverter was close to as sharp, but the lack of magnification effectively lost the benefit of that:
I didn’t try this test when I had the 100-400mm lens out.