I recently reported a quick comparison of the Sigma 100-400mm on my 5DSR with shots of a nearby flagpole. The results weren't quite as good as my Canon 100-400mm II. My local dealer got in another copy today and lent it to me for more extensive testing as I had a view to buying. I took shots of my favourite chart and compared it with my 100-400mm II and also my Sigma 150-600mm C on my 5DSR. Here are 100% crops from the centre at 10.5m distance. The Sigma (top) is not as good as its big brother at 400mm (bottom) or the Canon (middle).