It's a fine lens. But it would be sharper if you'd shot it with a 100-400 II and cropped a bit harder. Check the comparisons for yourself: https://www.the-digital-picture.com/...mp=7&APIComp=0 Plus, the Sigma is hugely heavier, has horrible handling (the worst of any long lens I've used, though the Canon 200-400/4 runs it close), inferior IS (any lens without Mode III IS is, in my view, automatically inferior to any other remotely similar lens with it), is approximately three elephants heavier, and only does f/6.3.

Is 1/3d or a stop important? My word it is.

The Sport was a clear upgrade on the 100-400 Mark I (so long as you could live with the weight, the handling, and the f/6.3), and it may very well be the best low-cost answer for Nikon owners (the Nikkor 80-400 seems rather friendless despite its spectacular price; however the 200-500, low price notwithstanding, is apparently pretty good and might give the Sigma a run for the money) but I strongly encourage any Canon owner to go for the 100-400 II if the big iron is out of reach. Excluding the $10,000+ monsters, the 100-400 II is simply the best lens I have ever owned or used. More than likely, it will be years before any competitor matches it.

For people wanting 600mm, please do not be seduced by the lure of the cheap f/6.3 zooms. They are perfectly OK lenses (even if not the best choice in that price bracket) but they are not remotely, not by any stretch of the imagination, in the same class as the big 400, 500, and 600mm f/2.8 and f/4 primes. If you really want the reach, raid the piggy bank a bit harder and look for something like a second-hand 500/4. Yep, it's even heavier than a Sport hurts your pocket too, but it is easier to hand-hold and vastly more capable. We are not talking an incremental difference here, the gap between them is a chasm.