Sigma 15mm f2.8

I use this lens in conjunction with my Nexus housing and the 4" dome port made specifically for that lens. With this set-up you can get really wide angle of view at a really close -focus. It can basically focus on the dome and is great for close focus wide angle

I use this lens with the 8 inch domeport from Ikelite and the results are great. What I most like about it are it's compact size and weight. It easily fits somewhere with the rest of my gear. My only regret is when you compare it to a 10 or 10.5mm lens there is a notable difference. The 10mm and 10.5mm will get you even closer to wideangle subjects. Perhaps not always a good thing for marinelife, but a better choice for wideangle coral and reef formations and/or wrecks. The Sigma 15mm is a great performer and nice and sharp, but looking back I prefer an even wider lens.