Chromatic Aberration

Chromatic aberration is an important photographic phenomena to understand. It might seem difficult, but it is simple enough. It is seen in photos as magenta and blue-green fringes produced by the lenses. Chromatic aberration can be created in two ways: 1. The lens does not focus the different 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.

The first thing to grasp is refractive index, so let us briefly explain what that is. Light changes its direction when it passes through a medium like the glass of the lenses. For example light may hit the lens at a 90 degree angle, but leave the lens at an 80 degree angle. Chromatic aberration arises because the different colors of light have different refractive indexes. For example blue might 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. Since green is in-between red and blue it is used to focus the lens. Thus the red and blue are slightly out of focus which creates the magenta (red+blue) 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 creates 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.

Both types of chromatic aberration produce color fringes, but of a different sort. Longitudinal aberration shows as magenta fringes around objects and is spread evenly 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. The so called achromatic lenses are by far the most popular with minimal chromatic aberration. Superacromatic and apochromatic lenses virtually eliminate color errors, but they are not common. 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 chromatic 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.

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. 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 underlying color is usually eradicated. 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.

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