Lens flare

Lens flare refers to a phenomenon wherein light is scattered or flared in a lens system, often in response to a bright light, producing an undesirable effect on the image. This happens through unintentional image formation mechanisms, such as internal reflection and scattering from material imperfections in the lens. 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. These mechanisms differ from the intended image formation mechanism, which depends on rays from the refraction of the image itself.

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). Visible artifacts, usually in the shape of the lens iris, are formed when light follows a pathway through the lens that contains one or more reflections from the lens surfaces.

Flare is particularly caused by very bright light sources. 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-quality optical systems, and for 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, although it does reduce contrast.

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.

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