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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).
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.
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.
The spatial distribution of the lens flare typically manifests as several starbursts, rings, or circles in a row across the image or view. Lens flare patterns typically spread widely across the scene and change location with the camera's movement relative to light sources, tracking with the light position and fading as the camera points away from the bright light until it causes no flare at all. The specific spatial distribution of the flare depends on the shape of the aperture of the image formation elements. For example, if the lens has a 6-bladed aperture, the flare may have a hexagonal pattern.
Such internal scattering is also present in the human eye, and manifests in an unwanted veiling glare most obvious when viewing very bright lights or highly reflective surfaces. In some situations, eyelashes can also create flare-like irregularities, although these are technically diffraction artifacts.
When a bright light source is shining on the lens but not in its field of view, lens flare appears as a haze that washes out the image and reduces contrast. This can be avoided by shading the lens (the purpose for which lens hoods are designed). In a studio, a gobo or set of barn doors can be attached to the lighting to keep it from shining on the camera. Modern lenses use lens coatings to reduce the amount of reflection and minimize flare.
When using an anamorphic lens, as is common in analog cinematography, lens flare can manifest itself as a horizontal lines. This is most commonly seen in car headlights in a dark scene, and may be desired as part of the "film look".
Deliberate use 
A lens flare is often deliberately used to invoke a sense of drama. A lens flare is also useful when added to an artificial or modified image composition because it adds a sense of realism, implying that the image is an un-edited original photograph of a "real life" scene.
For both of these reasons (implying realism and/or drama) artificial lens flare is a common effect in various graphics editing programs, although its use can be a point of contention among professional graphic designers. Lens flare was one of the first special effects developed for computer graphics because it can be imitated using relatively simple means. Basic flare-like effects, for instance in computer and video games, can be obtained using static or animated starburst, ring, and disc textures that are moved according to the position of the light source. More sophisticated rendering techniques have been developed based on ray tracing or photon mapping.
Lens flare was typically avoided by Hollywood cinematographers, but when filming Easy Rider, Laszlo Kovacs was forced to modify a camera car for his Arriflex, which resulted in numerous lens flares as he shot motorcycle footage against Southwestern U.S. landscapes.
J. J. Abrams, the director of the 2009 version of Star Trek, used this technique. "I wanted a visual system that felt unique. I know there are certain shots where even I watch and think, "Oh that's ridiculous, that was too many." But I love the idea that the future was so bright it couldn't be contained in the frame." Many complained of the frequent use, Abrams admitted it was "overdone, in some places."
David Boyd, the director of photography of the sci-fi Firefly series, desired this style so much (harking back to 1970s television), that he sent back the cutting-edge lenses which reduced lens flare in exchange for cheaper ones.
Other forms of photographic flare 
Filter flare 
The use of photographic filters can cause flare, particularly ghosts of bright lights (under central inversion). This can be eliminated by not using a filter, and reduced by using higher-quality filters or narrower aperture.
Diffraction artifact in digital cameras 
One form of flare is specific to digital cameras. With the sun shining on an unprotected lens, a group of small rainbows appears. This artifact is formed by internal diffraction on the image sensor, which acts like a diffraction grating. Unlike true lens flare, this artifact is not visible in the eyepiece of a digital SLR camera, making it more difficult to avoid.
Severe lens flare in a CCTV camera, showing both visible artifacts and reduced contrast and color saturation
High-end lens flare rendering using a recent technique
Lens flare used to capture details of too bright motive (partial solar eclipse).
Lens flare commonly associated with the use of anamorphic lenses which are often used in the production of films.
See also 
- Star Trek, a 2009 science fiction film that used lens flares to give the film a stylized look
- "Space Oddity", the first music video from space performed by Chris Hadfield in May 2013.
- Bokeh, a source of circles around out-of-focus bright points, also due in part to the internals of the lens.
- Diffraction spike, a type of lens flare seen in some telescopes
- Anti-reflective coating, used to reduce lens flare and produces the red and green colors common in lens flare.
- Evening, Martin (March 2000). Adobe Photoshop 5.5 for Photographers: A professional image editor's guide to the creative use of Photoshop for the Macintosh and PC, page 335. Focal Press. ISBN 0-240-51591-9.
- Albee, Timothy; Steve Warner and Robin Wood (January 2005). Essential lightwave 3D 8: the fastest and easiest way to master LightWave, page 434. Wordwave Publishing. ISBN 1-55622-082-0.
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- Keshmirian, Arash (August 2008). A physically-based approach for lens flare simulation (Masters Thesis Publication No. 1451802),. University of California, San Diego.
- Keith Phipps (2009-11-17). "The Easy Rider Road Trip". Slate.com. Retrieved 2009-11-19.
- "J.J. Abrams Admits Star Trek Lens Flares Are "Ridiculous"". Io9.com. 2009-04-27. Retrieved 2012-12-06.
- Česky. "Firefly (TV series) - Wikipedia, the free encyclopedia". En.wikipedia.org. Retrieved 2012-12-06.
- Filter Flare, Paul van Walree 2003–2010
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