Vortex Optics Flip Cap Optic Covers The hinged flip cap optic cover makes it a snap to keep fine optics protected and flips completely back so the cap lies flat. Simply slip the rubber cover over the objective or ocular end of your optic. Size 4 fits over a variety of optics with these outside diameters (OD): 35-40 mm | 1.4-1.6 inches Sold one per package.