The glass surface of the Canon Anti-Fog Eyepiece AE-B1 is coated with a special polymer to prevent fogging. When the Anti-Fog Eyepiece is attached to the eyepiece of the binoculars, it prevents eyepiece fogging. The anti-fogging is useful in the following cases:- In low temperatures when your breath, etc., can cause the eyepiece to fog up.- When the eyecups are pressed against your eyes for a prolonged period and fogging is prone to occur.- This is an useful accessory for the 15x50 and 18x50 Canon IS All Weather binoculars.