- Lens flare
Lens flare is the light scattered in lens systems through generally unwanted image formation mechanisms, such as internal reflection and
scatteringfrom material inhomogeneities in the lens. These mechanisms differ from the intended image formation mechanism that depends on refraction of the image rays. For good optical systems and most images, flare is a secondary effect that is widely distributed across the image and thus not visible. But when an image includes a very bright lightsource, flare generated by a bright image region can have enough intensity to become very visible. The light produced by flare mechanisms superimposes broadly across the image, adding light to dark image regions and reducing image 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
apertureof 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 (e.g. specular) surfaces. In some situations, eyelashes can also create flare-like irregularities, although these are technically
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 doorscan 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.
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 designersFact|date=September 2007. Lens flare was one of the first special effects developed for computer graphics because it is the result of relatively simple optical principles. During the mid- to late-1990s, it was a popular graphical effect for computer and video games, and is now accompanied by other more complex atmospheric effects that add a greater sense of realismFact|date=September 2007.
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.
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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.
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