The Vortex E-10 Defender Flip Cap Eyepiece (E‑10) is a rugged, spring-loaded flip-up eyepiece cover made to protect the rear (ocular) lens of a riflescope. It is built to fit eyepieces with an outside diameter of about 40–46 mm (1.57–1.81 inches), which means it’s compatible with most Vortex riflescopes except a few specific 1-inch tube Viper models (such as the VPR-M-01BDC, VPR-M-04BDC, and VPR-M-03BDC).

Designed to be virtually indestructible, the Vortex E-10 Defender Flip Cap fits snugly on the eyepiece and uses a snap flat spring mechanism that flips the cover out of the way when the scope is in use, keeping the shooter’s line of sight unobstructed. Some versions include multiple stop positions so the cap can rest vertical, at 45°, or flat back when opened, offering flexibility in how it sits relative to the scope.