Like at least some folks, I am trying to decide between the new Canon EF 100-400 II and the Sigma 150-600 Sport. There is really too little information out there now for the comparison of these with regards to image quality (though I am guessing that mindless religious fervour might have already decided the issue for some; if I shared that sentiment, then things would certainly be easier for me here!). Which makes things difficult to decide right now, because for me, reach and image quality are of paramount importance, with portability etc. being very secondary.
My main application will be wildlife and nature. I have been shooting for a couple of decades now, but my photography to date has primarily been overwhelmingly close-ups (rainforest herps: used to be a field herpetologist) and landscape. My longest lens is the EF 70-200L f/4 IS, but I now probably spend more time on the wide-angle side of things, with the TS-E 24mm f/3.5 and the Zeiss 21mm f/2.8 being my favorites (the former increasingly over the latter in recent times). I am trying to get into the wildlife side of things, and it looks like I picked a good year for it given the new lenses introduced this year!
Anyway, after poring over MTF charts and specs and sample images (more from the Sigma
than the Canon, which are very few and none of which are full-resolution, unpost-processed originals), here are some of my thoughts:
- The Canon (at 400mm) is going to have higher acutance (fine-detail resolution) than the Sigma at 600mm in the (full-frame) corners and edges at all apertures, though contrast will be not significantly different. Toward the center, the two will not be significantly different. (Stupendously speculative, based on comparison of cross-manufacturer MTF charts, and I will not argue with anyone who scoffs at this methodology or conclusion!)
- The Canon + 1.4xTC is going to be not significantly different than the Sigma at 600mm in both acutance as well as contrast across the frame when stopped down (Canon at f/8 or f/11, Sigma at f/6.3), and might, in fact be a little worse. (As above, stupendously speculative, based on comparison of cross-manufacturer MTF charts, and, again I will not argue with anyone who scoffs at this methodology or conclusion!) Of course, with the Canon it is going to be center-point AF only on some bodies and no AF on others.
- The Canon is going to have faster AF and better IS. (Somehwat speculative, but, even though I cannot even pretend that this is based on evidence, this one I am a little more confident about: just call me a Bayesian, i.e. I am happy placing priors on hypotheses and have these updated once the evidence is considered!)
- The Canon is (obviously and measurably) lighter and more compact, allowing it to be easily used in contexts where the Sigma will be awkward enough to be discouraging, e.g., event photography, family outings, fun travel, and so on.
- The Canon's compactness and weight as well as (speculative) better IS will make it preferable for backpackers or folks who otherwise eneed to travel light with the option of ditching the tripod/monopod (small aircraft, kayaks, crammed safari), or for folks who primarily do a lot of hand-held photography (e.g., BIF).
- In the wildlife domain, where "reach is king" and there is usually not much of interest in the corners and edges (or when using crop cameras), the Sigma will be preferable for those who do not mind schlepping the extra bulk and weight of not just the lens, but also a monopod/tripod + head (one might argue that if one is willing to schlepp all that, then for a few pounds more, one might as well schlepp a big gun like the 500 f/4 or 600 f/4, but those few extra pounds are going to cost you $8K extra).
- In the landscape domain (and yes, I am one of those that considers every focal-length a "good" landscape focal length; one of my pet peeves is the use of "landscape lens" and "wide-angle lens" interchangeably), the Canon will be a clear winner, due to its superior corner and edge acutance, and better filter flexibility (that hood-window to turn the polarizer is *nice*).
- [ADDED] For dragon-flies, butterflies, pond/back-yard wildlife: the Canon wins by its fantastic MFD and magnification factor, coupled with compactness and hand-holdability.
Full disclosure: I have both lenses on pre-order, so do not (yet!) suffer the real "I do not have it therefore it sucks" or "I have it so it is the best" syndrome. One the other hand, I, of course, have my own pre-existing biases (or priors if you want to get all statistical!). I think Canon has been knocking stuff right out of the park with their recent zooms, from the 70-200 f/2.8 II to the 24-70 f/2.8 II to the recent 16-35 f/4. By all accounts these are game-changers in that they challenge the conventional wisdom of primes are always better optically than zooms: these "generation 2" zooms are measurably equal or equivalent to any prime (or Canon prime, at least) throughout their focal lengths. On the other hand, Sigma has, until recently, had a very poor history. If this was still the case, then there would be no question. However, everything seems to have changed radically in the last couple of years, from the manufacturing to QA to service (this is not a subjective impression: it is verifiable fact). And, with the Global Vision rebirth, they, too, appear to be pushing out game-changing winners (their 50mm rivals the Zeiss Otus when stopped down one stop).
... ). The full review (not broken up into posts) can be seen 
