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.
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 enter the lens at 90 degrees, but from a different angle. Here the individual colors focus on the same sensor plane, but not at the same spot, thus the red image will be larger than the green and blue image, and blue smaller than red and green.This also creates 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.
The two types of chromatic aberration produce different kinds of fringes. Longitudinal aberration produces 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 image 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. Superacromatic and apochromatic lenses almost eliminate color errors, but they are not common. Chromatic aberration can be seen on film, but is most pronounced on digital images. 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.
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 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. 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 original color is usually suppressed. 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.
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 enter the lens at 90 degrees, but from a different angle. Here the individual colors focus on the same sensor plane, but not at the same spot, thus the red image will be larger than the green and blue image, and blue smaller than red and green.This also creates 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.
The two types of chromatic aberration produce different kinds of fringes. Longitudinal aberration produces 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 image 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. Superacromatic and apochromatic lenses almost eliminate color errors, but they are not common. Chromatic aberration can be seen on film, but is most pronounced on digital images. 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.
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 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. 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 original color is usually suppressed. 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:
This text is a rewrite of this: chromatic aberration I and this: chromatic aberration II. This article, Chromatic Aberration in Photography is available for free reprint.
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