Purple fringing

Severe purple fringing can be seen at the edges of the horse's forelock, mane, and ear.

The larger context of the above cropped image, from a Fujifilm Finepix S5200 camera.

In photography, and particularly in digital 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, since chromatic 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.^[1] 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.

[edit] 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 and interpolation artifacts (almost all CCDs require considerable processing)
• Stray ultraviolet light
• Stray infrared light
• Image bloom from overexposure
• Leaks between cells of the CCD

[edit] 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.^[2]

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.^[3]

[edit] References

1. ^ Sir David Brewster (1833). A Treatise on Optics, First American Edition, Philadelphia: Carey, Lee, and Blanchard, p.76. 
2. ^ Gary Nugent. "The Ultraviolet Filter". great-landscape-photography.com.
3. ^ Gary Nugent. "Photoshop Technique: Remove Purple Fringing". great-landscape-photography.com.

[edit] Other sources

• The Dreaded Purple Fringe (Pictureline News, vol 2 no 7, July 2004)
• "Chromatic Aberration" (Vincent Bockaert, DPReview.com) with good purple fringing example and a microlens conjecture
• Sony 828 Purple Fringe Analysis (Tom Fiddaman)
• Removing Purple Fringing (Scott Mendham, PC World Australia, 29 January 2004)