Depth meter Spirotechnique.
Picture using a Sony A7 in a Nauticam housing with a Nikon Nikonos 80 mm f/2.8 lens with one extension tube, taken à -20 m at Meyzieu indoor dive center. Higher blow up were done using the Nikon optical macro photo complement. Test shots with two Inon Z-240 strobes. The power of both was set to a different value.