I'm using a Sigma 15mm F2.8 EX DG with a Canon 5D in a Nauticam housing with a 4.3" dome and am having problems getting it to focus, auto or manual. The Nauticam chart recommends a 10mm extension, which I always assumed was just to prevent the lens sunshade from hitting the dome; I cut off the sunshade, so I'm not using an extension.
I've also read of using a 1.4 extender, which I don't know anything about. Does anyone have experience with this same setup and knows exactly what's required? Thanks!