It's a removable closeup lens for macro work. UW film shooters have been using it for a while and they seem to love it. People looking for more versatility for shooting an underwater DSLR setup might want to give it a try. I'm going to. They make models for Ike ports and Nexus ports
See this page for details:
http://www.nexusamer.../Misc/Misc.html
Cheers
James
