What is chromatic Aberration?

by Suzanne Britt
Chromatic aberration might sound very abstract, but it is quite easy, really. Imperfections in the camera lenses create the fringes, that are seen as magenta and blue-green fringes. It comes in two varieties: 1. The different colors do not focus on the same sensor plane. 2. The individual 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.Chromatic aberration is caused by the refractive index of glass, so let us first look at 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. Red might leave the lens at 81 degrees, while blue leaves at 79 degrees. This produces what is known as longitudinal chromatic aberration where you get thin magenta fringes. 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.

Transverse chromatic aberration arises when light does not reach the lens at 90 degrees, but from a different angle. 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. Chromatic aberration is hard to avoid, since it is in the nature of light, but of course lens manufacturers do their best to eliminate it.

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. However, lenses can be designed in many ways. The so called achromatic lenses are by far the most popular with minimal chromatic aberration. More rare are the superachromatic and apochromatic lenses, that virtually eliminate chromatic aberration. Digital images tend to show more chromatic aberration than film for some reason. This may be because the sensors are more sensitive to ultraviolet and infrared light, which are at the outer edge of the spectrum where aberration is most pronounced.

Software can fix cromatic aberration. Longitudinal chromatic aberration is somewhat corrected by sharpening the red and blue channels; the green channel is used to focus the image and is sharp. Transverse chromatic aberration is satisfactorily corrected by radially enlarging the blue channel image and radially reducing the red channel image.

Purple finging is a different kind of chromatic error. 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. Purple fringes are sensor errors, whilst chromatic aberrations are lens errors. Purple fringing is not a simple geometric error like transverse chromatic aberration, but is an overflow of light from the brightly illuminated sensor to its neighbors; hence it is very difficult to correct with software. Also the real color is usually lost. Software can 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.

 

About the Author:
Sources for the article are this: chromatic aberration I and this: chromatic aberration II