Wet diopter

I just came home from 12 days in Indonesia, Ambon to Raja Ampat. One nice thing about having so much luxury of time aboard the boat is that I was able to do some testing and experimenting, something never possible on a commercial shoot that has to get knocked out in a couple of days on location.

Since there was so much macro life to shoot, I was particularly interested in the external wet diopter. I shoot Seacam, so my diopter was the Seacam Wet Two, but there were a couple of Woody's diopters onboard being used on Subal systems, and I found my observations below are generally indicative of their product as well:

1. The magnification difference between no diopter and the Wet Two is significant. To test accurately I shot my 100 macro lens on a Canon 1DsMKIII (full frame) and manually dialed the lens out to minimum focus. The wider shots of the little sea apple and nudibranchs shown here represent full 1:1, lens racked out all the way and then camera moved forward until accurate focus achieved. Then I added the Wet Two (that's the beauty of these close-up lenses, they can be added and removed while underwater) and then moved closer until focus popped. You can see the relative enlargement they actually do provide. Quite impressive really.

2. The position of the front of the lens relative to the rear of the flat port glass is a massive variable. i tried to simulate this topside with shots of my watch face. You'll see the first shot is 1:1, minimum focus no diopter. The second shot is minimum focus with Wet Two and lens right up next to the rear of the macro port glass. The third shot, the one with all the smearing and optical aberrations at the edges, is with the lens maybe 2 inches from the rear of the glass.

It is important to note that macro lenses can be fairly casual about how they fit behind a flat port. They just have to not vignette and they'll work. But, to properly use a wet diopter, the front of the macro lens must be very, very close to the back of the glass on the macro port. Of course, this is easier with an internally focusing lens constantly at one point. But even with an extending lens, like the old style 60mm and 105mm micro Nikkors, the macro port extensions need to be designed so that the maximum lens extension (coincidentally, minimum focus) falls in proximity to the port glass if effective use of wet diopter is a goal.