Chromatic aberration is an important photographic phenomena to grasp. It might sound difficult, but it is easy enough. It is seen in photos as magenta and blue-green fringes produced by the lenses. It comes in two varieties: 1. The lens does not focus the individual colors on the same sensor plane. 2. The different colors produce images of different size. In the following article we will look in depth at the phenomena of chromatic aberration and how to avoid or solve it.

First we have to understand what refractive index is. When light passes through a medium, for example the glass of the lenses, the angle of the light changes. For example light may hit the lens at a 90 degree angle, but leave the lens at an 80 degree angle. The problem is that the different wavelengths of light have different refractive indexes. For example blue could leave the lens at 79 degrees while red might leave at 81 degrees. This difference will create thin magenta fringes known as longitudinal chromatic aberration. The sensor focuses on the green channel and chromatic aberration causes the blue and red to be slightly out of focus, which creates the combined magenta fringes.

When light does not reach the lens at 90 degrees, but from an oblique angle, you begin to get transverse chromatic aberration.. In this case the different colors focus evenly, but not at the same spot. This causes the red image to be larger than the green and blue, and the blue the smallest of them all.This also produces colored fringes, but now both a magenta and a blue-green one. It is in the interest of lens manufacturers to avoid chromatic aberration, but since it is in the nature of light, it is hard to eliminate.

You get different kinds of fringes for each kind of chromatic aberration. Longitudinal aberration shows as magenta fringes around objects and is distributed uniformly throughout the image. Transverse aberration is absent at the center of the image, but grows in intensity towards the corners. Longitudinal chromatic aberration is most pronounced in wide aperture lenses. It can be reduced by using a small aperture. Transverse chromatic aberration is most pronounced in telephoto lenses. There are numerous lens designs. So called "achromatic" lenses have minimal chromatic aberration and are very popular. More rare are the so called "superachromatic" and "apochromatic" lenses, that virtually eliminate chromatic aberration. Digital images tend to show more chromatic aberration than film for some reason. One explanation is that the sensors are more sensitive to ultraviolet and infrared light, which are at the outer edge of the spectrum where aberration is most pronounced.

Chromatic aberration can be fixed with software. By sharpening the red and blue channels, one can somewhat correct longitudinal chromatic aberration; the green channel is used to focus the image and should be sharp. Transverse chromatic aberration can be satisfactorily corrected by radially enlarging the blue channel image and radially reducing the red channel image.

A different kind of chromatic error is the dreaded purple fringe. It appears along hard contrast edges when photographing something against a hard back light, or when photographing a light source against a dark background.The purple fringe invades the dark area. We saw that chromatic aberration is a lens error; purple fringes on the other hand are sensor errors. It is very difficult to correct purple fringes with software since it is really an overflow of light from one sensor to the surrounding ones, and is not a simple geometric error like transverse chromatic aberration. Also the real color is usually suppressed. Software can thus reduce the color of the purple fringe to a grayish tone. At best the local color is not completely eradicated by the purple fringe and can be reconstructed.