Wide open at 1.8 to 4.0 aperture a ring appears at the outer age of the picture and as you stops down to f4.5 it disappears, seems something not right inside the lens.
開口1.8から4.0まで写真のエッジに円が付きます。f4.5に絞ると消えますので、原因はレンスの問題かと思います。