In
photography
Photography is the art, application, and practice of creating durable images by recording light, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is employed i ...
(particularly
digital photography
Digital photography uses cameras containing arrays of electronic photodetectors interfaced to an analog-to-digital converter (ADC) to produce images focused by a lens, as opposed to an exposure on photographic film. The digitized image is ...
), purple fringing (sometimes called PF) is the term for an
unfocused purple or
magenta
Magenta () is a color that is variously defined as pinkish- purplish-red, reddish-purplish-pink or mauvish- crimson. On color wheels of the RGB (additive) and CMY (subtractive) color models, it is located exactly midway between red and blu ...
"ghost" image on a
photograph
A photograph (also known as a photo, image, or picture) is an image created by light falling on a photosensitive surface, usually photographic film or an electronic image sensor, such as a CCD or a CMOS chip. Most photographs are now create ...
. This
optical aberration is generally most visible as a coloring and lightening of dark edges adjacent to bright areas of broad-
spectrum
A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors ...
illumination, such as
daylight
Daylight is the combination of all direct and indirect sunlight during the daytime. This includes direct sunlight, diffuse sky radiation, and (often) both of these reflected by Earth and terrestrial objects, like landforms and buildings. Sunlig ...
or various types of
gas-discharge lamp
Gas-discharge lamps are a family of artificial light sources that generate light by sending an electric discharge through an ionized gas, a plasma.
Typically, such lamps use a
noble gas (argon, neon, krypton, and xenon) or a mixture of th ...
s.
Lens
A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...
es in general exhibit axial
chromatic aberration
In optics, chromatic aberration (CA), also called chromatic distortion and spherochromatism, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the wa ...
, in which different colors of light do not
focus in the same
plane. Normally, lens designs are optimized so that two or more (at least three for
apochromatic
An apochromat, or apochromatic lens (apo), is a photographic or other lens that has better correction of chromatic and spherical aberration than the much more common achromat lenses.
Explanation
Chromatic aberration is the phenomenon of differen ...
lenses) wavelengths of light in the
visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called '' visible light'' or simply light. A typical human eye will respond to wave ...
focus at the same plane. Wavelengths very different from those optimized in the design process may be severely out of focus while the reference colors are in focus; this
axial chromatic aberration is usually severe at short wavelengths (violet). Lens performance may be poor for such wavelengths in other ways too, including an increase in
flare
A flare, also sometimes called a fusée, fusee, or bengala in some Latin-speaking countries, is a type of pyrotechnic that produces a bright light or intense heat without an explosion. Flares are used for distress signaling, illumination, ...
due to anti-reflective
coatings
A coating is a covering that is applied to the surface of an object, usually referred to as the substrate. The purpose of applying the coating may be decorative, functional, or both. Coatings may be applied as liquids, gases or solids e.g. Powder ...
also being optimized for the expected wavelengths.
Most film has relatively low sensitivity to colors outside the visible range, so light spread in the near
ultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiatio ...
(UV) or near
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from arou ...
(IR) rarely has a significant impact on the image recorded. However,
image sensor
An image sensor or imager is a sensor that detects and conveys information used to make an image. It does so by converting the variable attenuation of light waves (as they pass through or reflect off objects) into signals, small bursts of cu ...
s used in digital cameras commonly are sensitive to a wider range of wavelengths . Although the lens glass itself filters out much of the UV light, and all digital cameras designed for color photography incorporate
filters
Filter, filtering or filters may refer to:
Science and technology
Computing
* Filter (higher-order function), in functional programming
* Filter (software), a computer program to process a data stream
* Filter (video), a software component th ...
to reduce red and IR sensitivity , the chromatic aberration can be sufficient for unfocused violet light to tint nearby dark regions of the image . Bright cloudy or hazy skies are strong sources of
scattered violet and UV light , so they tend to cause the problem.
The term ''purple fringe'' used to describe one aspect of chromatic aberration dates back to at least 1833.
However, Brewster's description with a purple fringe on one edge and a green fringe on the other is a ''lateral'' chromatic aberration. A general defocus of the shortest wavelengths resulting in a purple fringe on all sides of a bright object is the result of an ''axial'' or ''longitudinal'' chromatic aberration. Quite often, these effects are mixed in an image. Axial chromatic aberration is more subject to reduction by stopping down the lens than lateral chromatic aberration is, so the purple fringing can be very dependent on
f-number
In optics, the f-number of an optical system such as a camera lens is the ratio of the system's focal length to the diameter of the entrance pupil ("clear aperture").Smith, Warren ''Modern Optical Engineering'', 4th Ed., 2007 McGraw-Hill Pro ...
: a larger f-number (smaller
aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane.
An op ...
) reduces axial aberration.
Other explanations
Purple fringing is usually attributed to
chromatic aberration
In optics, chromatic aberration (CA), also called chromatic distortion and spherochromatism, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the wa ...
as described above. Other attributed causes of purple fringing in digital photography include many hypothesized sensor effects:
*Digital noise in dark areas
*Image processing and
interpolation
In the mathematical field of numerical analysis, interpolation is a type of estimation, a method of constructing (finding) new data points based on the range of a discrete set of known data points.
In engineering and science, one often has a ...
artifacts (almost all CCDs and CMOS require considerable processing)
*
Stray ultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiatio ...
and/or
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from arou ...
light
*Image bloom from overexposure of CCD sensor (not applied to CMOS)
Mitigations
Commonly advocated methods of avoiding purple fringing include:
* Avoid shooting with a wide-open lens in high contrast scenes
* Avoid overexposing highlights (e.g. specular reflections and bright sky behind dark objects)
* Shoot with a strong UV-cut filter
Post-processing to remove purple fringing (or chromatic aberration in general) usually involves scaling the fringed colour channel, or subtracting some of a scaled version of the blue channel, or other blue-channel tricks.
References
{{reflist
Science of photography
Digital photography
Visual artifacts