Sigma makes a 12-24mm zoom lens made to cover a full frame sensor. It is slow (4.5-5.6) but otherwise looks like it could be useful underwater. Does anyone use this lens, or have any knowledge of its quality? Should I just assume it would have the same soft corner issues of the Nikon 14-24? I'd love to hear what others think.

On a 1Ds2, the corners are soft until f8 topside. Underwater, it's just soft. Corners are acceptable at f8 with the 1.25x 1D2. That's my copy of the lens. There are also variations in the QC for the lens so test several copies of the lens and choose the sharpest one. At 12 on FF, it's amazingly wide with very little distortion. It does flare a bit so be careful with the source of light placement.
Center sharpness of the lens is nice through f stops. Only the corners suffer. With underwater, shooting in open water, the soft corners are usually blue so you won't notice it as much.

1D2N f11 1/250 15mm

I had an excellent copy (stolen in 2005) which I purchased at a local camera shop. I tested 7 lenses there and picked the best one. All of the WA photos in my Bali gallery were taken w/ this lens on a 1DII camera. I never got a chance to try it on a FF camera although I would really like to.

Regards,
James

However, soft corners can be a pretty significant issue depending on what you are shooting. If you shoot anything other than subjects in mid water, this could be very problematic. If the lens can accept a diopter, that would be your best bet to help sharpen up the corners. Also going to a larger dome port with a larger internal radius would help.

The lens does not accept a diopter. Basically this lens has the same "issues" as Nikon's 14-24 zoom. It's an ultrawide lens - and just about any lens this wide is going to have a huge curved field of view to focus on. Hence the corners are out of the DOF range.

Above water the lens performs surprisingly well - check out some of the reviews and samples. If you can't do a pano, then this lens is the next best thing.

Cheers
James

Well Schneider and Tiffen have 138mm +2 filters we can cut to fit the lens

What housing/port/tube are you using that is large enough to handle the Sigma 12-24mm?
Does it allow for zoom?

I used it with a Seacam housing and a custom made gear I made by cutting and Ikelite gear and rubber-banding it onto the lens. Cheapest custom gear I've ever used :-)

Cheers
James

I've improved on it. It's now taped to the lens so it doesn't move. Rubber bands still move.