Flare is a optical phenomenon that occurs when unwanted non-image-forming light enters a camera lens and reflects, scatters, or bounces between the internal lens elements, the lens barrel, and other surfaces within the camera body before reaching the film or image sensor. The result is a reduction in image contrast, a washing out or veiling of shadow areas, and in more pronounced cases the appearance of distinct bright streaks, polygonal shapes, or ghost images overlaid across the frame.
Flare is most commonly caused by shooting towards or at an angle to a bright light source, such as the sun, artificial lighting, or any other intense point of light that falls within or just outside the angle of view of the lens. When light strikes the curved glass surfaces of a lens at certain angles it can reflect internally rather than passing cleanly through, creating secondary and tertiary reflections that degrade the quality of the image. The polygonal shapes sometimes seen in flare are caused by light reflecting off the blades of the lens aperture diaphragm, and their shape corresponds directly to the number of aperture blades in the lens.
Several factors influence the degree to which a lens is susceptible to flare. Lenses with a greater number of elements provide more internal surfaces from which light can reflect, increasing the potential for flare. The quality and type of anti-reflective coatings applied to the lens elements plays a significant role in controlling flare, with modern multi-coated lenses offering substantially better flare resistance than older uncoated or single-coated designs. Zoom lenses, which contain more elements than most prime lenses, can be more prone to flare for this reason.
The most effective practical measure for reducing flare is the use of a lens hood, which shades the front element of the lens from light sources outside the frame that would otherwise strike the glass at an angle and cause internal reflections. Keeping lens elements clean and free from fingerprints, dust, and smears also helps to minimise flare, as surface contamination can scatter light and worsen the effect. While flare is generally considered an unwanted artefact to be avoided, it is occasionally used deliberately by photographers and filmmakers as a creative device to convey atmosphere, warmth, or a sense of light and energy within an image.