The Dreaded Purple Fringe

There seems to be an awful lot of discussion going on in the digital camera world about the dreaded purple fringe. This phenomenon exhibits its self as an out of focus purple ghost image rimming parts of a digital picture.

This event, purple fringing, was thought to have been brought about strictly by Chromatic Aberration in the lens design. CA occurs when one to all three of the primary wavelength colors (RGB) do not focus at the same destination point. Less expensive or less exacting lenses exhibit this behavior regularly. It is easy to see with a single element lens, such as a magnifying glass. Tip the glass so that the light rays are not passing directly through its center and at some point a rainbow effect will be cast upon the reflecting surface. Modern camera lenses utilize multiple lens elements in an attempt to prevent this problem. Lens creators also use special low dispersion glass (LD, UD or ED designations), special lens coatings, aspherically ground elements and occasionally fluorite elements to counter act design problems and to eliminate CA in their more expensive lines. Most of the low end digital cameras ($100-$200) have been accused of having simple and inexpensive lenses which are more likely to be plagued with CA, thus more purple fringing. However, with some of the new 8 megapixel cameras, with great lenses, reporting purple fringing, researchers began looking deeper than just the taking lens.

The imaging sensors became the next suspicion. The light imaging sites on the sensor, often wrongly referred to as sensor pixels, are set down into the silicon chip material like small buckets. The imaging sites must physically be separated from each other to avoid bloom or bleed over. Bloom is blown out highlights, like in a fantasy dream sequence, which leave temporary white trails. Bleed over is color smearing and polluted color intensity between sites. To funnel light down into the separating buckets each site is covered with a microscopic lens. With eight million tiny lenses, all within three eights of an inch, there is bound to be stray or inadvertent reflected light. So test shots were made in an attempt to determine if CA of the site lenses was the culprit. It does not appear that the site lenses alone are the problem.

Tests have been conducted to see if too narrow of site spacing (too tightly packaged) was the problem. Testers wanted to know if voltage was leaking from one image bucket to the next because of narrow spacing. Testing began on the algorithms used by each manufacturer to process their image information to see if they were problematic. Tests were also conducted to see if extra or unfocused UV or Infrared was causing site pollution. None of these symptoms alone seemed to be the problem.

To date there is no clear single item that is responsible for purple fringing. We do know that is most likely to happen when apertures of greater than f/4 are used, when a wide angle of 28mm (or equivalent) or wider is used and when the subject to background contrast is high. Since it is most likely to happen under those conditions, avoid that combination if possible. To remedy some of these problems it seems to be most advisable to use a lens hood or shield the lens from light raking directly a crossed it. Under exposure and low light shooting conditions will also aggravate the situation. Image processing attempts to find or add something where really there is nothing. Even when shooting film, large dark areas are problematic. The digital noise in dark areas will lend itself to the digital purple dread. Avoid image sharpening in Photoshop where there are large areas of dark values. Some of the fringe may be reduced in Photoshop by image> adjust> color balance and adjusting down the blue channel enough to camouflage the purple.

There seems to be much disagreement about and many technical reasons, linked together in some random way, as to why purple fringing occurs. Most of the problems seem to be as a result of individual camera designs and there is little we can do about changing those factors. Use the tips given to improve your odds of not being purpled. After all is said and done, I would rather have a picture of my sweethearts with a little fringing than no picture at all. Enjoy your photography and have a great summer.

July 2004