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. 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 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, and is reduced by using a lens
hood or other shade. For good optical systems and most
images, 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.
Manifestation 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. 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 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, Harrison Arnold 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. Costume designer Rita Riggs purposefully
would sometimes dress Bea Arthur in Maude in a red that would flare to make
a statement, to match Maude's personality as a "red flag".
Director J. J. Abrams added numerous lens flares to the 2009 film Star Trek
by aiming powerful off-camera light sources at the lens. He explained: "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, that he sent back
cutting-edge lenses that 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. 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.
Gallery See also
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.
References