An optical artifact caused by light entering a lens from an undesired direction. Optical systems rely upon most light traveling a set, 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 and appear as bright patches in the image.
There are three types of lens flare. They can appear in any combination, but pictures which use the third effect are more likely to use it exclusively:
Circles or polygons of various pale colors and sizes situated on a line pointing towards the bright light source. These shapes are images of the aperture of the camera's diaphragm, 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.
Bright streaks or lines radiating from the light source. Can appear with a rainbow gradient. In a picture with multiple light souces, often all streaks from all lights are parallel or perpendicular. The mistaken belief that this and diffraction spikes are the same thing is widespread. As a result, artists often render these effects indistinguishably.
Anamorphic lenses produce a characteristic type of lens flare similar to the second, except that horizontal streaks are strongly enhanced, being much brighter, longer and thinner than streaks in other directions (if there even are any). Long time use of these lenses by the film industry has led to this kind of lens flare being very strongly associated with the movie-going experience; it is very common for this effect to be simulated to make something feel cinematic.
