So, home-made solution to an antique problem, although variations might be useful in some cases still....
I am shooting with a rather old Sony A55 SLT, from the very first generation 'translucent' mirrored series that is currently capped by the A99 full frame (which I use above water). The SLT series has electronic viewfinders. I have an Ike housing but elected to use Inon Z240 strobes vs. Ike 125/160/161 for their smaller size.
Unfortunately, in these very first SLTs, the A33 and A55, Sony 'helpfully' had the viewfinder mimic predicted exposure results to give the photographer a "what you see is what you get" type intuition. Unfortunately, in an underwater housing with manual strobe sync, that means it 'helpfully' turns black with settings like ISO200, f11, 1/160th underwater because the camera doesn't know you're getting significant light boost from a strobe. AF module works fine - but you're shooting blind. Using the EVF tunnel or the rear screen makes no difference. UW photogs are a tiny market but this same effect hampered studio photogs with radio triggered flash, so for the next generation SLTs (starting with the A65) Sony offered a 'setting effect off' option in the menu so the EVF brightness is always tied more to ambient light not exposure prediction. (Same option is available in all the A6xxx and A7 mirrorless offerings now). Will help me when I upgrade but for now....
The only other fix is TTL - as long as the camera knows a strobe is indeed present, f11, 1/160th or so isn't going to black out the viewfinder. I do have a DS51 which I use when shooting macro (and have a fiberoptic half-ring flash arrangement I printed up posted about elsewhere). Finally getting a dive trip again this year to a location I want to shoot wide angle with the Inons again, so decided to do something about my dilemma.
Hence - a cover plate to use the DS51 as the light source for optical sync to the Inon z240 sensors, but without having the light from the DS51 influencing the shot by just letting it blap out all over. Printed a cover and 'adaptors' for the sensor caps on the Inon to accept any TOSlink audio S/PDIF cable. At like $6-8 from Monoprice vs. the much more expensive underwater manufacturer fiber optic cables, I can afford to keep a few spares around expecting the inevitable corrosion around the FO tips (since these were clearly never intended for saltwater immersion). Cable in the picture is a Monster TOSlink cable I had laying around and look forward to drowning (purchased many moons ago before I knew what a**hats they were as a company....)
First pic of connection (two FO links available at the strobe face, only one currently in use) below.
