Here's an interesting underwater lens I came across. Spanish makers of underwater photography equipment Saga Dive have integrated two different diopters into one configurable lens. This lens has a M67 screwmount and screws on the front of your flat port. By rotating the levers you can choose between a +5 diopter, a +10 diopter or a combination to make a +15 diopter for the optimum macro performance. This is a 'wet mate lens'.
Aluminum body (6082)
Hard anodized coating: 40 microns
Two glasses BK7 flat with anti-scratch and anti-reflective coating
Three lens, with group of two elements
Weight: 700 gr.
Resistent depth: 100 meters
1 group of lens and another group of two achromatic lens
Function: By means of external levers, with a rotating movement toward the center we can position a lens +5 or +10 and the sum of both, for getting a +15 lens, thus eliminating the need to carry more than one lens underwater or flipping between lenses.