Chromatic aberration sounds difficult, but it is really very easy. 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 individual colors do not focus on the same sensor plane. 2. The different colors produce images of different size. Here we will take an in-depth look at what chromatic aberration is and how to avoid or solve it.
First we have to understand what refractive index 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 produces what is known as longitudinal chromatic aberration where you see thin magenta fringes. 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 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.
A special 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. 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.
First we have to understand what refractive index 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 produces what is known as longitudinal chromatic aberration where you see thin magenta fringes. 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 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.
A special 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. 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.
About the Author:
This article is based on this: chromatic aberration I and this: chromatic aberration II. This article, What is Chromatic Aberration? is available for free reprint.
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