The following clip isn't that long but should give you an idea of what the lens, camera and dome combination are capable of. I will definitely be trying this again! It was worth cutting off the hood::
Last year I purchased a used Ikelite DSLR housing and 8" modular dome port to house a Canon 5D2. For my first dive trip I used the Canon 17-40L with the #5510.24 port extension and the 8" modular dome. In the end it worked out okay but I didn't care for the edge sharpness - especially for video at the wide (17mm) focal length. I did try a +2 diopter and I think it helped a bit but I had a lot of problems focusing at/around infinity so I took it off after a day of diving. What I did like about the 17-40 is that is has a relatively close focus distance and so stills shot at 40mm were very sharp and worked well with a single strobe. But wide angle just wasn't working all that well. Here is a link to a gallery of photos (all underwater all taken with the 17-40L). Note: many of these photos are cropped, some quite significantly:
I had seen some very nice footage shot with the Canon 14mm L version II. But that lens is way over my budget!
So I searched around and found a Rokinon 14mm f2.8 lens used on Craigslist for $220. It was in decent shape. By f4 it is very sharp and for my copy by f5.6 it is crazy sharp and has no noticeable vignette on the full frame 5d2. It is a very fun lens to use. You can shoot straight into the sun with little or no flaring! The moustache shaped distortion is immediately obvious if there are any horizontal straight lines. Easy to fix for stills but need to be aware of for video.
Of course, I really wanted to take it underwater and thought it might fit in the Ikelite system - nope! It was so close, but the built in hood of the lens is too wide to fit through the ikelite port extension tube. So I tried taking the lens apart a few times (to cleanly remove the hood) but never did figure it out. Along the way I declicked the aperture ring - which I thought I would like - but it had no damping and the throw is so short I put that tiny bearing back in.
From earlier measurements, I knew that the lens would fit through with the lens hood removed. So, given the lens was cheap, I cut the hood off with a dremmel tool. I first removed the font element (easy), stuffed the lens with lint free cloth to keep the particles from entering the rest of the lens and then simply cut off the lens hood.
After this the lens fits quite easily through all the ikelite port extension tubes. And I am able to use the Ikelite focusing gears! FYI - This is a manual focus lens so the focus gears working has been very handy.
The first port extension tube I tried was the 5510.24. There was a lot of vignette from the modular dome. I'm believe this port extension would have worked focus-wise though. However, when I purchased the used Ikelite kit, it also had the slightly shorter #5510.22 and the very short #5510.11. So next I tried the #5510.22 on a local lake dive. The conditions were crappy but it seemed to work okay. I really struggled with focusing though as it was so dark. Although I did capture this image when were were exiting (this is through the dome but above water!)
Then two weeks ago I was down in Ambergis Caye for a week. Unfortunately we were blown out for the majority of the trip so I only got in three dives. On the first dive I tried the #5510.11 (very short port extension). I think it was just too close to the lens to focus properly. Only the very center of the image was properly focused.
On the only other two dives I switched to the #5510.22 extension tube. On the first dive I struggled with focus. I was confused and thought I needed to stick a land based hyperfocal focus point - but that really didn't work. On the second dive I had more time and refocused with live view. This time the results were a lot better! When I disassembled the rig at the end of the day I noted that the focus was very close to the minimum focus distance. Perhaps with a dome port and in water, the focus is on the virtual image right behind the dome?
For all the dives I used Magic Lantern. It has such an easy whitebalance menu system (just two button clicks). But in the end I wasn't overly pleased with the results. Because 14mm is so crazy wide on a full frame I had to hold the white card so close that often it had shadow on the card. But I'm also wondering if the Magic Lantern whitebalance doesn't use a very full range of tint?
I should also note that this lens is manual aperture. So I just set the aperture at the click stop between f8 and f12 - I'll call it f10. The shutter was set at 1/47 and I ended up using iso 100, 160 and 320 for the dives. I probably should have set the aperture at f12. You will see that the water was pretty hazy and a bit dark but it worked.
Also, I had the cinemascope crop marks turned on for all the dives as my goal was to create a 2.35:1 video when finished. So the sample clip is cropped this way.
1.) When shooting stills, there is a very noticeable vignette from the dome port. If you plan on shooting lots of stills you will need to crop to get rid of vignette. But I like wide angles with very landscape aspect ratios so no problem here. And at 20MP, you can crop forever on the 5D2.
2.) When shooting video, the full video frame (16:9) has small - but just noticeable - corner vignetting. The average viewer may not notice it but anyone critical will call it out. So, cropping to a more anamorphic aspect ratio is probably best. But once again, this lens is crazy wide which lends itself to that type of presentation.
3.) Corner sharpness. I think it is quite good! The 17-40 was much softer. And I think with more dive time to get focus better I think the Rokinon / Samyang 14mm f2.8 will rival or even best the Canon 14mm L version II. Perhaps if placed behind a glass dome (vs the Ikelite acrylic) it might be even sharper?