It is probably useful to differentiate between soft corners and other optical aberrations.
George, if this statement is related to corner sharpness it is a curious one. DP-FE (and 2, 3, 4) is thicker and has a smaller radius of curvature than the acrylic optic Nauticam uses for the 7-14 port. Assuming the same placement, these factors would result in softer corners.
I think 7-14 can perform quite well if you are careful to shoot an f-stop of around f11. I like the lens for blue water subjects, but would choose a fisheye for most close focus wide angle on a reef.
Keep in mind that there are few (or no?) 35mm equivalent 14mm lenses that have a good reputation for underwater photography (except possibly Sigma 8-16 on DX). Zooming to 8 or 9mm makes a big difference if shooting more open f-stops with subjects in the corners.
Not so. The 7-14mm Nauticam dome is 6 inches...the Subal FE3 is an 8 inch dome.
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.
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.
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. 