I'm currently working with the 12mm in the 4.33" dome port. The corners are a little soft at f/8, but quite manageable, edge sharpness is as good as the 12-50mm behind the flat port (but the 12mm f/2.0 is significantly wider behind the dome). It's possible that this lens behind the 3.5" semi-dome would perform slightly better.
Recently i've been trying (dry) close-up lenses on the 12mm behind the 4.33" dome. The reason for this is that the 12mm doesn't focus down to 0.10m like the 8mm FE. Placing a 52mm +5 diopter on the front using a 46mm to 52mm adapter, allows much closer focusing (underwater the long edge is about 17cm, vs 37cm w/o a diopter). However, because of the lack of depth of field at the close focus, the strongly curved 4.33" dome results in rapid loss of sharpness away from the centre. I'm thinking the ideal solution for the 12mm will be the 3.5" semi-dome without the extension (as Nauticam recommends) but with a +X diopter -- i haven't seen this tried (yet).
That said, i'm very happy with the images from the 12mm behind the 4.33" dome. Corner sharpness is much better than i often see from big DSLRs with very large domes. I don't hesitate to use this lens. It's sharper in the centre and has better contrast underwater than the 12-50mm behind the flat port.
Anyway (12mm behind 4.33" dome corner sharpness test, f/6.3 no diopter):
Edited by coroander, 31 January 2014 - 04:40 PM.