Lens flare

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Photograph of NASA lunar lander containing lens flare. Besides the obvious flare around the Sun, the light artifacts at the bottom right are also caused by flare.

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. 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 light source, 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 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 that is apparent when viewing very bright lights or highly reflective (e.g. specular) surfaces.

Lens flare is extremely difficult to control when a bright light source like the sun is just outside the frame.

When the subject of a photo is the light source itself, lens flare can be a desirable and dramatic effect.

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 can be attached to the lighting to keep it from shining on the camera. Modern lenses use lens coatings to reduce the amount reflection and minimize flare.

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. During the mid- to late-1990s, lens flare was a popular graphical gimmick for computer and video games.

See also

• Bokeh, a source of circles around out-of-focus bright points, also due in part to the internals of the lens.

External links

• [Lens Flare] - Understanding flare and techniques to reduce it.

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