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I would have to agree with Phil , the Sigma I tried was rubbish off axis just for regular photography let alone for astro.
I now have the Nikkor 14-24 with adapter for the 5D great lens ( even wide open ) as I keep telling people but then they go out and buy the Canon 16-35 and then complain about its performance .
If you stop the 16-35 down it should be ok , but be careful of the Sigma.
You get what you pay for I guess.
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