- Purple fringing
In
photography , and particularly indigital photography , purple fringing is the term for an out-of-focus purple ghost image on a photograph. Images taken with high-contrast boundary areas involving daylight or gas discharge lamps are particularly susceptible, sincechromatic aberration is worst for the shortest wavelengths that a camera is sensitive to (violet and/or ultra-violet light).The term "purple fringe" to describe one aspect of chromatic aberration dates back to at least 1833, before the invention of photography. [cite book | author = Sir David Brewster | title = A Treatise on Optics | edition = First American Edition | publisher = Philadelphia: Carey, Lee, and Blanchard | year = 1833 | pages = p.76 | url = http://books.google.com/books?vid=OCLC00718218&id=QUMIAAAAIAAJ&pg=PA76&lpg=PA76&dq=purple+fringe+chromatic ] 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 .Other explanations
Purple fringing is usually attributed to
chromatic aberration , 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:
*Chromatic aberration in each CCD cell (microlenses)
*Digital noise in dark areas
*Image processing andinterpolation artifacts (almost all CCDs require considerable processing)
*Strayultraviolet light
*Strayinfrared light
*Image bloom from overexposure
*Leaks between cells of the CCDMitigations
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. [cite web | title = The Ultraviolet Filter | author = Gary Nugent | work = great-landscape-photography.com | url = http://www.great-landscape-photography.com/ultraviolet-filter.html]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. [cite web | title = Photoshop Technique: Remove Purple Fringing | author = Gary Nugent | work = great-landscape-photography.com | url = http://www.great-landscape-photography.com/remove-purple-fringing.html]
References
Other sources
* [http://www.pictureline.com/newsletter/article.php?id=311 The Dreaded Purple Fringe] (Pictureline News, vol 2 no 7, July 2004)
* [http://www.dpreview.com/learn/?/Glossary/Optical/chromatic_aberration_01.htm "Chromatic Aberration" ] (Vincent Bockaert, DPReview.com) with good purple fringing example and a microlens conjecture
* [http://www.sd3.info/pf828/Sony_828_Purple_Fringe_Analysis.html Sony 828 Purple Fringe Analysis] (Tom Fiddaman)
* [http://www.pcworld.idg.com.au/index.php/id;154451015;fp;2;fpid;1585691688 Removing Purple Fringing] (Scott Mendham, "PC World" Australia, 29 January 2004)
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