I saw this "dark ring" issue with my 5D3 today.
I was shooting daffodils etc. in sunlight against dark backgrounds. I was shooting RAW + JPG.
Got them home, and saw dark borders around bright parts of flower petals etc.
Since I'd been shooting RAW too, I tried re-processing in camera and turned sharpness down to minimum and created another JPG.
Looked at that on the computer and the dark border was gone.
So I've reduced the sharpness setting on the camera now.
Could you post a shot with the dark ring?
That dark border around the bright "shaft" thing - it wasn't really there and vanishes when you reduce the sharpness setting in the camera.
Canon 5D3 , 70-200mm f/2.8 L IS II , 24-105mm f/4 L IS , 50mm f/1.4 , 85mm f/1.8 , EF 2x III , Samyang 50mm T/1.3
Canon EOS M3 , EF-M 18-55 f/3.5-6.3 , EVF-DC1
Olympus OM2-SP , 50mm f/1.2 , 55mm f/1.2 , 50mm f/1.4 Silvernose , 135mm f/2.8 , 28mm f/2.8