Greetings -

I'll start by telling you all that I'm not the photographer in the family, but have
been outfitting She Who Is with a new camera (D200), housing (Titan) and a
pile of lenses:

Tokina 10-17
Sigma 17-35
Nikkor 60mm
Nikkor 105VR

plus for topside

Nikkor 18-200VR
Tokina 80-400

She also has a few other lenses from her older Nikon cameras (a 2020,
which we still have, and a D70, which we recently sold). This is also her
first housed digital SLR - she has been shooting various Olympus cameras
(3030, 5050, 7070) under water for years.

In any case, my quandary is the Sigma 17-35. Light & Motion doesn't support that
lens, but I'm wondering if it could be made to work anyway. I've asked them
directly and the answer I got was "we don't know".

While it's not critical, it would be nice to be able to use it. Light & Motion does
support the Nikkor 17-35 lens.

The Nikkor is 82.5mm in diameter, and the Sigma is 83.5mm, so there's a small
chance that the Titan's gear for the Nikkor would work.

However, the Nikkor is 106mm in length, while the Sigma is 88.7mm, so I'm guessing
that a different extension ring would be required. There appears to be 4 different
extension rings that Light & Motion offers, creatively named Extension Ring [1234].
The Nikkor lens requires Extension Ring 3.

Any advice appreciated - which could include "use the 17-35 topside and just shoot
the 10-17 under water".

Your best option would be to measure the diameter of the zoom ring on the lens and send that info to LMI. They can tell you if they have a gear that fits.

Next, it's very hard to find which extension ring is the correct one. You need to find the nodal point of the lens (which is somewhere inside the lens) and use enough extension to push the center of curvature of the dome out so that it hits the nodal pt of the lens. The easiest way to do this is to get them to send you a variety of rings and take test shots w/ each one. Then send back the 3 that don't work.

Cheers
James