I've been happy with the low light, high ISO performance overall. I've shot quite a few pics of the kids inside with ISO up to 6400 and they turned out great with some noise reduction.

I haven't tried to shoot wildlife with any EF glass yet. I did try shooting some eagles in flight in poor light with the EF-M 55-200 and it didn't perform well, but under those conditions my 5DsR and 300 f/2.8 II would not have produced any keepers either. It was just all I had at the time and I figured it would be a good test.

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