What is "lens flare"

n. A stray patch of brightness in a photography resulting from aberrant refraction or reflections within the lens due to an exceptionally bright light source, sometimes one just outside the image proper. Lens flare is usually unwanted, but can be exploited for artistic effect.

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. 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.

For good optical systems and most images (which do not have a bright light shining into the lens), flare is a secondary effect that is widely distributed across the image and thus not visible, though it reduces contrast. Lenses with large numbers of elements such as zooms tend to exhibit greater lens flare, as they contain multiple surfaces at which unwanted internal scattering occurs.