The EP-15 Hydrophobia Eyepiece for Canon 6D/5D/5D Mark II/Rebel T3 from Think Tank Photo is a required watertight accessory for your Hydrophobia rain cover. It allows you to see through the camera's eyepiece once the rain cover is installed. The slide-on EP-15 eyepiece is CNC machined out of Delrin acetal resin and features high-density foam padding. It clicks into place so you know it's sealed and secure.