We have examined the reason why the 13mm is so much better and apart from the coating the dome of the 13mm is not a typical dome but a real lens with varying thickness at different areas.
We did some testing in Croatia. I was at 60m with a customized Subal ND3s and it was waterproof with the 13 mm RS lens. The pictures were remarkable especially regarding corner sharpness. The small dome of the Nikonos could easily surpass the sharpness of large 9” domes at larger apertures but still offering depth of field characteristics of small domes at smaller f-stops – and that with full FX frame. A dream came true.
Reinventing Nikonos - RS Lenses On Digital SLRs
Comparison of video lights from Subtronic, Keldan and Hartenberger and Subal's new prism llive-view finder
A synopsis of strobes from Subtronic, Seacam and Sea & Sea and their performance and handling underwater