Thanks for taking the time to test and post these, Stephen.
DX clearly performs the best (12-24mm), particularly with a dioptre, which is as expected. I have also seen it performing very well with a dioptre behind some of Seacam's smaller domes too.
And the Canon 16-35 Mk2 is certainly much better than either Nikon lens (at least without dioptres). Did you test many port extension rings with the two lenses to determine which is best - as you said above - after testing the 16-35mm Mk2 you asked Seacam to make a new extension ring specifically for it?
Certainly from my tests posted above, the addition of a dioptre made a massive difference for corner sharpness with the 17-35mm (I realise dioptres are not practical with the 16-35mm and 14-24mm). I look forward to seeing these results in comparison with the 16-35mm.
I disagree with Tom and Craig about zooming the lenses to the same points. Of course that would be a fairer comparison - and stopping the full frame cameras down an extra stop would be fairer too! But the reality of a photographer using any of these lenses underwater is that they would not stop zooming out if they needed to. They would use the range of the lens that is fitted to their camera. The tests need to be relevant to actual shooting and it is important to know how the lens you attach will perform (I realise Tom, that you don't dive with your cameras - so can't zoom while shooting - the rest of us do).
Although I do take Tom's point about F5.6 being very open. I used it above because I felt it was the widest I would typically go with the aperture - and obviously the most likely to show problems. I also tested a F14 - where differences are less clear.
Anwyay, Stephen, thanks again for the tests. I'll post something on the front page.
Alex
p.s.. And for some reason I didn't note that Seacam made a D300 housing before, which is also good news.