Not so. The 7-14mm Nauticam dome is 6 inches...the Subal FE3 is an 8 inch dome.
Actually is so. You have to differentiate the dimensions of the segment used from the dimensions of the sphere it was cut from. Subal FE domes have an external ROC of 200mm, and internal of 186. From memory this is cut down a bit, but that doesn't directly influence corner sharpness. The Nauticam dome is in fact larger, but I don't have exact measurements. It is also thinner, but I'm confirming how much.
I do remember reading a good pool test done by another Wetpixel member of the IQ resulting from larger domes vs smaller, aggressive curvature domes. The larger domes gave what to me seemed a better optical result. I've found the same in my own tests of the several types of Subal domes that I have...including a new very aggressive "compact fisheye" port (FE100) that I bought to try with my favourite UW lens, the old Minolta 7.5mm MD.
With rectilinear lenses, my experience shows that the greatest contributor to sharp corners is the domes size, but thickness is important enough not to be ignored.
There are many optical/physical factors that operate...indeed interact...in complex ways as the diameter of the dome glass changes; i.e nodal placement, virtual image(s) proximity, peripheral distortions, etc.
As far as I'm concerned the bigger the dome's diameter the better the optical performance.
True if comparing the same thickness in every case I've ever tested.
The merits or otherwise of the Nauticam domes is all a bit irrelevant to me now...I've sold my Nauticam GH2 housing in anticipation of the new GH3 which should be announced in a few days time. What little information there is about the GH3 suggests it should be a superb camera...even without a hack (i.e. H.264 @ 70 mbps I-frame out of the box). It will be weather sealed and have a larger pro quality body...hence will not fit the current Nauticam GH2 housing.
I'm quite excited for this camera as well.