I was surprised at the CA, too, but didn't seem to get it on most shots with the diopter and lens, so I'm not sure how to explain it! :glare: All of the pygmy shots against black background had it. Could the fact that I was shooting him at 1/250th and f/32 (max sync speed and minimum aperture) have had anything to do with it?
Hi Bruce, I don't shoot Canon but the Wet 2 and similar focal length, in my case D2x with 60mm, has been my go-to macro set up for about two years now. I like it for subjects as small as pygmies or as big as anemonefish (hey, we all shoot them....why not admit it

) but not anything big enough to make me step back a foot or more, like a medium sized frogfish.
In my experience CA has not been an issue because I suffered CA as bad as that in your seahorse image when shooting a couple pipefish sticking their heads out of some white coral (open background, not black,SS @ 1/80, and aperture at f25) in PNG in 2006. So I did a little layman's experimentation to see if it was something as simple as the physical setup of my camera/housing rig or something inherent in the diopter's performance.
What I found is that the alignment of the lens to the flat port, and the Wet 2 being pushed all they down against the flatport glass, is critical to the avoidance of CA. It took me a while to figure this out because without the wet diopter it never really mattered if the flat port was not effectively perpendicular to the lens axis, but I can both create CA and eliminate/mitigate it by dictating whether or not the Wet 2 is effectively perpendicular to the lens axis. This probably doesn't matter to other wet type diopters because they are not achromats, but I suspect the fact that both the Wet 2, and the 60mm, or in your case 100mm, lens are achromats, it does matter when using the Seacam diopters.
Now, not a bit of that is based on science but it is what I found when figuring out how to make the combo as productive as possible. Perhaps someone with an optical background could weigh in.