In Guadalupe I was very happy with the performance. I used a +3 dioptre on the lens (I tend to favour the +3 for pelagics and the +4 for reef wide angle with these lenses) a port extension ring and a Subal FE2 port. This review is about shooting real world subjects, and the shark images with the 17-35mm are all very acceptable. I was satisfied that wide rectilinear was working for me on FX and did not even try any other port/dioptre combinations.
Shark photography is not the toughest challenge for corner sharpness. Firstly, most shots do not have any important corner detail. Moreover, corner softness is accentuated by short camera to subject distances, which you would get with reef CFWA, rather than shooting pelagics. So for those who like a bit of measurebating, this thread in the forums has some photos of the cage grid. Normally I would have used the +4 dioptre for such shots, but in any case, these shots are not too bad at all. The corners suffer more with fringing than sharpness and the problems only manifest right in the corners at FX.
There is one important further feature of the D3 that is worth mentioning in respect to dome performance. The D3 has an in-camera AF fine-tune feature, which can remember specific settings for certain lenses. I tried setting a slightly closer focus (than the true distance) for the 17-35mm, to bring the plain of focus forward slightly relative to the curved virtual image to help corner sharpness. I felt this improved matters, but I was not able to achieve any quantifiable results, mainly because the 17-35mm was already working well. When I own a camera with this feature I will experiment further.
Despite getting technically acceptable images with the 17-35mm, I have to admit that I just don’t like the pincushion look of the shots. I am a fisheye guy. Give me cuddly, curvy barrel distortion any day over spikey, pointy pincushion. To me the underwater world looks so much better that way. So most of the time I put the fisheye on.
Optically the Sigma 15mm was superb. There is no doubt that fisheye lenses perform very well on FX, and if like me that is what you like to shoot, then you will be very happy. Up in Port Hardy it was without a doubt the lens to have. The full 180 degree view is definitely what is required for wide angle photography. It allowed me to photograph the colourful walls with a diver or a colourful foreground with the bull kelp towering above as a background.