I finally just got a Tokina 10-17 lens for my D90 and brand NEW D7000. The kit is going in an Aquatica housing with an 8" dome port. The guy at the UW camera place says I need Aquatica Extension ring 16.5mm for the 10-17, yet I thought that was not needed for the fisheye. This is my first fisheye, so I am looking for some help on whether and/or why the extension ring is needed. Thanks in advance for the help.