An optical artifact caused by light entering a lens from an unwanted direction. The image-forming capability of optical systems rely on most light traveling a known, coherent path through it from a particular direction, usually straight ahead. Light from other directions of sufficient intensity can reflect and scatter in pathways other than the intended one, which appears as bright patches of various shapes and sizes in the image.
Lens flare generally appears in three different ways. Each can appear by itself or with another in any combination, although pictures with the third type are more likely to use it exclusively:
Bright streaks radiating from the light source.
Circles or polygons of various pale colors and sizes centered on a line pointing to the light source. These shapes are multiple images of the camera's diaphragm aperture, also known as an iris. Cameras with 5-bladed irises will produce pentagons, 6-bladed hexagons, etc., with more blades producing a progressively more circular shape. These can appear even if the light source is outside the field of view.
Anamorphic lenses are a special type of lens that have been used extensively in movie cameras since the 1950's. They produce a characteristic lens flare similar to the first, except streaks are strictly horizontal. Streaks in other directions are either not present or much fainter. Because of its use in the film industry, for most people this type of lens flare has a very strong association with movies, and this effect is often simulated to impart a cinematic quality.
Type 1 (streaks, with prominent halos (see below))
Type 1+2 (streaks and hexagons)
Type 1+2 (streaks and circles)
Type 3 (anamorphic type)
The following are NOT lens flares:
Rainbow-colored arcs that partially or entirely encircle the light source. These are most likely halos. Halos are not camera artifacts; like rainbows, they are caused by light being reflected and refracted by water droplets or ice crystals in the atmosphere.
bokeh. Bokeh is affected by the shape and size of a camera's iris in the same way as the second type of lens flare. However bokeh does not appear in a line, instead each spot corresponds to a single, out-of-focus light source.
"Love bubbles": In anime, manga and associated illustrations, to give a scene a "lovey-dovey" or "dreamy" feeling, often the artist draws many translucent circles in the background and around the edges of the image. While sometimes appearing similar to lens flare, they have no corresponding light source. These can even appear in dark pictures with no bright light sources at all.
