The Series 4000 Focus Gear from Aquatica is for use with the Nikon AF Micro 105mm f/2.8D lens, which works with 18430 Mini Macro Port. Simply mount the gear on your lens focusing ring, 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
Nikon AF Micro 105mm f/2.8D lens, which works with 18430 Mini Macro Port