This is a custom spring-loaded, squeeze-style lens cap designed specifically for this lens. It is made from premium high-strength, heat resistant polymer with stainless steel hardware. It attaches to the front of the lens and grips the rubber edge like a standard lens cap. It can be attached or removed with or without the hood in place. It is flat on the front so it can stand on the end without tipping. The grips are easy to use even with gloves. This is the lens cap that should have been included with your expensive lens. Made by hand in Seattle, WA (not mass-produced).