I've been using a 5d Mark III for the past 2 years, and I've recently noticed that when I take a photo of a group of people, the ones in the top left of the frame always come out noticeably softer than the ones on the right or in the center.
This apparently does not depend on the lens used, I've tried both a 70-200mm 2.8, a 16-35mm f/4 or a 40mm prime. I get excellent sharpness in the middle and right parts of the frame, but the people on the top left appear as if they were a bit out of focus, regardless of the aperture used and even if I focus on them.
Is that a common issue? Not a deal breaker to me, but I wonder what could be causing it.