Shooting half and half pictures with a fish-eye lens can be successful when you use a small aperture and focus on the underwater section. At about f20 the above-water section will be included in the depth of focus. Apparently. But in my pictures the background seldom looks sharp enough if I am focused on a foreground subject, like a girl in a bikini. The distortion created by a fish-eye is also an issue for some subjects - like the aforementioned young woman.
Years ago I experimented with custom plano-convex diopters made in obscure strengths, like +3.3 for my 6" dome and +2.4 for my 8" dome. The "plano" aspect was meant to flatten the curvature of field but I found the B&W diopters were simply better.
On my 18mm lens I used split diopters for half and half shots in +3 and +4 strengths. Recently I have been playing with these again. One drawback was that once the diopter was mounted on the lens, you were committed to horizontals or verticals for the entire dive. No longer.
The Nauticam zoom gear I have has a sleeve about 85mm diameter. 3" drainpipe here in Thailand can have an internal diameter a shade less. Heat the end with a blow-lamp to soften it and you can push it over the Nauticam sleeve. You can pull it off afterwards, but it's a firm fit.
The 77mm filter holder I have has an outside diameter of about 80mm, so a 5mm long ring of PVC pipe glued in the end and sanded a bit acts as a strong push fit on the filter holder. The castellation cuts at the end allow for slight expansion.
Mount the filter in the pipe, then screw it onto the front of the lens then mount the camera normally in the housing. Paint the PVC with a black felt-tip pen to avoid reflections.
Ideally, have the dioptre mounted so that it works in the horizontal position with the filter almost tight so that only a 90 degree turn is required to put it in the correct position for verticals. As for useful dioptre strengths for half and halfs, that's another story!
The latest version of the Leak Sentinel arrived yesterday. For 200 euro it seems a good deal.
Apart, it looks like this:
The cap is removed, the red valve depressed to switch on a micro-switch on the circuit board, a hand vacuum pump attached and pumped until the flashing red LED turns to flashing green. This takes about 25 pumps with my Nauticam D800 housing. The cap is then replaced to keep water out.
If the light remains green, then there's no leak. After the dive, remove the cap, and push the red rubber valve to one side to allow air to enter. Press again to switch the circuit off.
To replace the battery, the three Allen-head screws must be removed, not a difficult job.
It is installed in the 16 x 1 threaded hole in the housing. I used some thread-lock to prevent it being unscrewed accidently.
I have a Nauticam D7000 housing, 2 bodies with less than 40000 shutter actuations on each, a Nikon 12-24,and Sigma 8-16 with gears for sale. My wife needs D800 cameras for weddings, so it doesn't seem sensible to have DX and FX of everything. PM me if you are interested.