Place this Ikelite Control Ring Gear over the front control ring on your Canon S120 camera before installing it in Ikelite's dedicated #6242.12 underwater housing. The gear is pressed into position around the lens, and can be programmed to control aperture, shutter, focus, and other functions. It is held in place by the three nylon hex head set screws that were supplied with your housing. These screws can be tightened with ease using the wrench that was also included. The control ring gear ships with the housing as a standard accessory, and is offered here as a spare or replacement part. You may find that you want to keep it on the camera even when you're not using the underwater housing, as the gear makes it easier to rotate the control ring.