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.
The first thing to grasp is refractive index, so let us briefly explain what that is. When light passes through a medium, for example the glass of the lenses, the angle of the light changes. Light hits the lens at 90 degrees, but leaves the lens at 80 degrees (just an example). Unfortunately 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.
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.
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 correct 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 should be 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.
The first thing to grasp is refractive index, so let us briefly explain what that is. When light passes through a medium, for example the glass of the lenses, the angle of the light changes. Light hits the lens at 90 degrees, but leaves the lens at 80 degrees (just an example). Unfortunately 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.
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.
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 correct 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 should be 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.
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Sources for the article are this: chromatic aberration I and this: chromatic aberration II
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