If you want to use your lens zoom function while taking photographs underwater, this zoom gear should be used. This Sea & Sea Zoom Gear is used to manually focus the Sigma Macro 70mm F2.8 EX DG lens when used in a Sea & Sea housing for any camera. Sea & Sea precision machined zoom gears are manufactured from either anodized aluminum or durable plastic polymer. Both versions are corrosion resistant. Easy to install in housing.