A feature or shape at the focal plane of a camera or other optical instrument that is not present in an actual scene, or an unfocused duplicate image that is overlaid upon a desired image. Ghost images, or ghosts, are caused by reflections from the surfaces of lenses or windows. Each glass surface divides incoming light into two parts: a refracted part that passes through the surface, and a reflected part that is turned back. If the reflected light is turned back again by reflection from another glass surface or a mirror, it may travel to the focal plane to form a ghost image. Ghost images may appear as an out-of-focus blur or smudge, a sharp circle or polygon with the shape of the camera iris or other aperture, or a false image of an object within a scene.