Most of the time purple fringing is a phenomenon that happens to a conventional digital camera lens. In other words purple fringing is a form of chromatic aberration at the camera lens level. Consequently, purple fringing may be seen through the entire image frame, which is different from regular chromatic aberration. Along the edges of high contrast subjects are the most affected, particularly when backlit.
The purple fringe label describes a single
chromatic aberration aspect that goes back to the early 1800's, before
photography was invented. However, Sir David Brewster's descriptions of purple fringe on an edge and green fringe on an other is classic lateral chromatic aberration which is general defocusing of the wavelengths which are the shortest which produces a purple fringe that occurs on every side of a brightly lit object becomes the consequence of an axial aberration or sometimes longitudinal chromatic aberration. Many times these effects are blended into an image. Reduction of axial chromatic aberration can often be accomplished by stopping the lens down than reducing lateral chromatic aberration can, so most purple fringing is a consequence of the f-number.
An example of purple fringing around the ears and top of the horses' head,
avoiding the back lighting would have prevented the purple fringing here
Purple fringing is most often recognized as a form of chromatic aberration, even though it is not apparent that all aspects of purple fringing is able to be described this way. Some other causes attributed to purple fringing occurring in digital photography comprise of many hypothesized sensor consequences:
Chromatic aberration occurring in every cell of a CCD (micro-lens)
Digital noise occurring in darker areas
Interpolation objects and Image processing (just about all CCDs require extensive processing)
The most common supported methods for steering clear of purple fringing consist of:
Don't shoot wide-open in high contrast situations.
Don't overexpose highlights (e.g. brilliant reflections with bright skys' behind dark subjects).
Use a Haze-2A filter or any other strong UV filter.
Use post-processing to eliminate purple fringing (and overall chromatic aberration) which most often consists of rescaling the color channel that is fringed, and/or removing part of a scaled portion of the remaining blue channel.
Apr 19, 2011