Another vote in favour of the much-maligned Canon. Maybe I've just got an unusually good copy? The Sigma is a fine lens, but it seems to me to wash colours out and give things a strange jaundiced cast which, being colour blind, I can't get rid of very effectively. In any case, and the Canon managed this, which I was fairly happy with:
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I mean... obviously it's not the sharpest lens in the world, but I don't think it's as soft as people make it out to be (that shot is @ 300mm, and it's not like you can't make out what kind of bird it is), and the autofocus is much better than the Sigmas I've tried; quicker and quieter than the Sigma equivalent(s). And I think it looks better as well, which is obviously very important also

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