Lens flare is the light scattered in lens systems through generally unwanted image formation mechanisms, such as internal reflection and scattering from material inhomogeneities in the lens. These mechanisms differ from the intended image formation mechanism that depends on refraction of the image rays. Flare manifests itself in two ways: as visible artifacts, and as a haze across the image. The haze makes the image look "washed out" by reducing contrast and color saturation (adding light to dark image regions, and adding white to saturated regions, reducing their saturation). Visible artifacts, usually in the shape of the lens iris, are formed when light follows a pathway through the lens that contains one or more reflections from the lens surfaces.
Flare is particularly caused by a very bright light sources either in the image – which produces visible artifacts – or shining into the lens but not in the image – which produces a haze. Most commonly, this occurs when shooting into the sun (when the sun is in frame or the lens is pointed in the direction of the sun), and is reduced by using a lens hood or other shade.