I suspect at f/8 you are overexposing the scene, and cannot see the edge darkening. Try shooting a dark wall rather than a light wall.
Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.
Benefits of registering include
I was naive not thinking that Thank you tursiops for the great helpI suspect at f/8 you are overexposing the scene, and cannot see the edge darkening. Try shooting a dark wall rather than a light wall.