This is a larger gear for the
Z-Drive. It attaches to Z-Drives purchased after Dec 8, 2015 via three included screws. This allows the Z-Drive to engage with smaller diameter lenses than the original gear allows. If you have an older version of the Z-Drive and want to upgrade to the newer Z-Drive to allow for the larger gear, email sales@zacuto.com. There is a $75 charge for the upgrade.