The Giant Octopus (Enteroctopus) filmed in East Sea of Korea.
This octopus may grow up to 3m(10ft) in length and weigh more then 70kg(154lbs).
All still image courtesy of JungKwon Park (Diver in this video).
I been testing 16-35mm f4 topside with f2.8 II. New f4 is sharper corner and have better lens coating (better sunball).
I couldn't try it behind dome but still if I don't have f2.8 then I would buy f4. IMHO, both lens will give you better sharpness and contrast then 17-40.