I had a Bigma that was very sharp in the past, then I picked up a 100-400L pretty cheap, so I did a mini-review back then of the Dust Pump against the Bigma. I found both to be very good IQ wise, and the slight nod went to the Canon due to weight, IS, and the ease of adding TCs and manually focusing over the Bigma. I decided to pick up this latest Sigma lens, since it has OS, and I was very optimistic that I would find it to be comparable to the Canon and my old Bigma.
Nice sunny day so that I would not have any issues with AF, and all hand-held at ISO 800. I sat on my porch chair and did these shots pretty quickly to get them all in the same exposure conditions. I shot large JPG for all shots, on a 1DMKIII. I ran a quick micro adjustment on the Sigma, I have had a much longer time period over the past months to tweak the 100-400 to the MKIII, so that may be a factor as well to some extent here on some of the discrepancies between the lenses. I took the filter off the Canon, so that it would not be a factor in this. Also, I didn't quite hit the correct zoom factor on the Sigma, as all I could do is get close to the markings on the barrel.
I have an action that I ran for each and every shot, so the post processing was the same for each. I do an auto-level, USM contrast step, overlay layer with high pass at around the 1.0 mark, then merge. No saturation steps or USM sharpening steps.
The Sigma is longer than the Canon by quite a bit, and won't fit in my bag if I keep it. The OS seems to be much better than the IS on the Canon, BUT only when it is not doing a strange little twitch from time to time, it was really bugging me. I could see it jerk from time to time, other times it was so locked in, it was uncanny. I think they have a little work yet on this version of OS. The color rendition is different than the Canon. It cannot utilize the 77mm filters either, like the Canon and so many of the other lenses, both Canon and Sigma.
The Canon seems to resolve more detail though at different points, at least with my copies. It could be that I have a very sharp 100-400 and a softer Sigma, it is hard to tell with such a new lens. It could be something to do with the contrast difference between the two lenses as well. The Canon is still lighter and smaller overall, but I really did like the zoom ring on the Sigma versus the push/pull/friction locking ring on the Canon. The Canon is easier to manually focus as well, but with age, I am sure like all other Sigma lenses, the focus ring will become much looser/smoother.
Overall, it seems to be a really good lens at its price point and its range, but I think Sigma needs to address the OS behavior, and it is still a very long lens, it reminds me a bit of the older 170-500mm lens. If I didn't have the Canon, I would seriously consider this lens as my defacto telephoto lens. I will play a bit more with both lenses, and one of the two will then go up for sale.