What is Purple Fringing?
Purple fringing is a phenomenon whose exact nature, though not existence, is still under debate. The phenomenon itself manifests itself as a purple edge that appears around objects in a photograph, typically under high-contrast conditions. Some think that purple fringing falls under the description of chromatic aberration, the distortion of color inherent in optical physics . While we may normally think of a camera’s optics as having the same effect on all frequencies and colors of light, this could not be further from the truth. The refractive index (a measure of the difference between the entry and exit angles of an incoming ray of light) varies based on the composition of the material that ray is passing through, but it is also dependent on the frequency of that ray. Under most circumstances, as the frequency increases, the refractive index also increases, causing each color to focus at a different distance. Chromatic aberration also occurs as a difference in the magnification of objects based on color; while the manifestation is different, the underlying cause is identical.
In the case of digital photography, it is commonly believed that a contributing factor in purple fringing is chromatic aberration in the microlenses used to focus light on the image sensor. Because these lenses are optimized for the green portion of the electromagnetic spectrum, red and blue light are accordingly distorted; the fringes thus created combine to form the appearance of a purple fringe. Whether due to user error, equipment, or shooting conditions, it can ruin an otherwise good photo. The question is, what can you do about it?
How Do You Stop Purple Fringing?
There are a number of proposed ways to deal with purple fringing, some of which occur before the fact, and some that you execute afterwards. Just as in the case of lens flare, if light is the source of the problem, then reducing the amount of incoming light should reduce the problem – however, unlike in that case, the phenomenon should always be there to some degree, whether you can detect it or not.
Another means of eliminating it is through post-processing. Photoshop is always an option (the high-contrast conditions under which the fringe occurs makes removal that much easier), though removing the fringe manually may add to both the boredom and frustration that users are experiencing; to cope with this, apparently, someone developed an action for Photoshop that is a "purple fringe killer".
You may also want to consider exposure bracketing, as well, since it doesn’t cost anything but a little time and memory space and may save you quite a bit of hassle!