Fogstop is a coating applied to the inside of the lens. It is not smooth but rough – seen through a microscope it looks quite fissured. This makes it impossible for moisture droplets to form a covering layer, i.e. to mist up the lens, impairing visibility. This could only occur if the humidity level was such that all the fissures are filled with water. But before this can happen, the stored moisture actually condenses, keeping the lens clear.