The Size 4 Flip Cap Optic Cover from Vortex is designed to be used for either the object lens or ocular of an optical system with an outside diameter between 35-40mm. The flexible rubber collar slides over the body and the matte black plastic cap folds down and snaps into place to cover the lens. Once closed, the raised disc inside locks the cover into place, helping to keep moisture and dust away from the lens. Two tabs on the cover make unlocking it quick and easy. A 270º spring-loaded hinge flips the cover back and holds it flat against the collar.