Purple fringing
Encyclopedia
Photography
Photography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...
, and particularly in digital photography
Digital photography
Digital photography is a form of photography that uses an array of light sensitive sensors to capture the image focused by the lens, as opposed to an exposure on light sensitive film...
, purple fringing is the term for an out-of-focus purple or magenta "ghost" image on a photograph. This defect is generally most visible as a coloring and lightening of dark edges adjacent to bright areas of broad-spectrum illumination, such as daylight or various types of gas discharge lamps.
Lenses in general exhibit axial chromatic aberration
Chromatic aberration
In optics, chromatic aberration is a type of distortion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have a different refractive index for different wavelengths of light...
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 3 for apochromatic lenses) wavelengths of light in the visible range focus at the same plane. Wavelengths very different from those optimized in the design process may be severely out-of-focus when the reference colors are in focus. Lens performance may be poor for such wavelengths in other ways too, including an increase in flare
Flare
-In culture and the arts:*Flare , an acrobatic move employed in b-boying, commonly known as breakdance, and gymnastics*Flare , a female hero who is a member of the League of Champions...
due to anti-reflective coatings 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
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
(UV) or near infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
(IR) rarely has a significant impact on the image recorded. However, sensors 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 most digital cameras incorporate filters to reduce IR sensitivity, the leakage can be sufficient for bright out-of-focus UV/IR light to tint nearby dark regions of the image. Typical digital sensors are particularly sensitive to near IR, and the Bayer filter
Bayer filter
A Bayer filter mosaic is a color filter array for arranging RGB color filters on a square grid of photosensors. Its particular arrangement of color filters is used in most single-chip digital image sensors used in digital cameras, camcorders, and scanners to create a color image...
blue and red filters typically leak more IR light than the green does, thus explaining the apparent color of the fringes.
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
F-number
In optics, the f-number of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter...
: a larger f-number (smaller aperture
Aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...
) reduces axial aberration.
Other explanations
Purple fringing is usually attributed to chromatic aberrationChromatic aberration
In optics, chromatic aberration is a type of distortion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have a different refractive index for different wavelengths of light...
as described above, although it is not clear that all purple fringing can be explained this way. Other attributed causes of purple fringing in digital photography include many hypothesised sensor effects:
- Digital noise in dark areas
- Image processing and interpolationInterpolationIn the mathematical field of numerical analysis, interpolation is a method of constructing new data points within the range of a discrete set of known data points....
artifacts (almost all CCDs and CMOS require considerable processing) - Stray ultravioletUltravioletUltraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
light - Stray infraredInfraredInfrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
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 Haze-2A or other 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.