All really good opinions and viewpoints. I never really thought of the compatability issues for the D850 - personally I'd rather go for something tried and tested for UW (e.g. choice between existing rectilinear and fisheye lenses), than to hope they'll bring out a new lens that'll solve everything.
Corner sharpness is something I've started paying attention to a lot more, and has been stated, big domes = pain in the a... at the airport. For me, the ease of packing and transporting is actually pretty fundamental, as I only bring my rig abroad, but I may start shooting in the UK at some point. Either way, it'll have to be transported!
I guess i'm in the position of 'measure twice, cut once' - so I think given the amount of research I've done into this upgrade, I'll be going for the D500 with the recommended tokina and 105mm nikkor macro lens.
The d500 body itself can be bought for £1300 with 3 yr UK warranty, and with a hugyfot housing that totals to about £4000. Not the cheapest, but a lot cheaper than £3500 for the D850 and an extra £3000 for the subal housing, totaling £6500. Couple that with dome/ports etc., then that's a £2500 saving, assuming the dome and ports are the same for both. Lenses would probably be more expensive on the FX lens.
So all in all, some of the benefits of having FX in general underwater would possibly be:
- Dynamic range: d500 has excellent DR
- ISO/noise: d500 is pretty good on the noise for higher ISO levels
- Continuous shooting without strobe: d500 has excellent burst/buffer
- High MP for prints: this is a tricky one, as the D500 has lower MP, but I don't be doing enormous prints, and I aim to crop as little as possible so this shouldn't be an issue
- Wider FOV vs DX: well, this isn't always an advantage under water, but definitely almost always on land
I'll be going for the D500 and hugyfot, but I need to carefully select my setup so it's easy to pack! I do however look forward to the next generation of cameras coming out after the D850. I feel the D850's been a bit of a game changer, and an incredibly exciting release - hopefully other manufacturers will follow suit and prices will go down and tech will go up.
@AdamHanlon: If I were to go for the d500 with hugyfot, do you feel it would be reasonably future proof (i.e. 5-10 yrs)? Or are there aspects of the D500 which you feel are technically inferior and is due for replacement soonish? From what I can tell, the D500 is a beast, and despite being DX, it has a lot of the FX qualities you'd expect in an FX camera, and even some features of the D5!