I agree with Clive on how to test it, I also have a Sigma Bigma and I find it is soft at the extreme end - 450 - 500mm unless you have very good light and are well stopped down - f8 + and 1/750 or faster for me - also agree that unless it is on a tripod or beanbag you will struggle. Personally however I'm not too bothered about softness in an image. I find it great for landscape and am not too much into birds. I would agree with Pete that for consistent sharp wildlife you need a prime but I find my Nikon 80-400 better than the Bigma.
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