I have a Sony A7RV camera in a Nauticam housing with a 140mm glass dome port, and am trying out a Canon EF 8-15 f//4 lens with a Sigma MC-11 adapter. I am very happy with the quality of the images this produces, but have a major problem. The Canon lens is supposed to have a minimum focal length of 140mm but my lens focuses on the inside of my dome port as well as any bubble on the outside. I reduced the recommended port spacer from 30mm to 20mm bringing the lens to within 30mm of the dome, which is not recommended, but this does not fix the problem. However on checking the minimum focal distance I find that the lens focuses at 17mm!! How is this even possible? Do any other users have the same problem? Is this a problem on my particular lens? I have tried using different focus modes, but this does not solve the problem. Obviously keeping the port in pristine condition and cleanliness is optimal but it is distressing to have a dive or two where it keeps focusing on that one particle of dandruff that inadvertently crept in. Have any other users experienced this problem.
