The Focus Gear from Aquatica is for use with the Canon EF 100mm f/2.8 lens, which works with 18430 Mini Micro Port. Simply mount the gear on your lens focusing ring, and connect it to the focus control knob of the underwater housing. Once beneath the waves, you'll be able to control the focus from outside the housing.
Compatibility
Canon EF 100mm f/2.8 lens, which works with 18430 Mini Micro Port