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The Focus Gear from Aquatica is for use with the Canon EF 100mm f/2.8 USM lens, which works with 18430 Mini & 18429 AF Low Profile 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
Canon EF 100mm f/2.8 USM lens, which works with 18430 Mini & 18429 AF Low Profile Macro Port