Chromatic aberration is an important photographic problem to grasp. It might seem difficult, but it is simple enough. It is created by the camera lenses and is seen as magenta and blue-green fringes in the photos. It comes in two varieties: 1. The lens does not focus the individual colors 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. 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. Unfortunately different wavelengths 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. Here the different 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 produces colored fringes, but now both a magenta and a blue-green one. Lens manufacturers go into great length to produce lenses that have a minimum of chromatic aberration, but since it is in the nature of light, it is hard to avoid.
Both types of chromatic aberration produce color fringes, but of a different sort. Longitudinal aberration produces 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 minimized by using a small aperture. Transverse chromatic aberration is most pronounced in telephoto lenses. However, lenses can be designed in many ways. So called achromatic lenses have minimal chromatic aberration and are very popular. So called superacromatic and apochromatic lenses virtually eliminate color errors, but they are not common. Chromatic aberration can be observed 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.
Chromatic aberration can be corrected with software. 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 can be 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. We saw that chromatic aberration is a lens error; purple fringes on the other hand are sensor 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 original 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.
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. Unfortunately different wavelengths 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. Here the different 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 produces colored fringes, but now both a magenta and a blue-green one. Lens manufacturers go into great length to produce lenses that have a minimum of chromatic aberration, but since it is in the nature of light, it is hard to avoid.
Both types of chromatic aberration produce color fringes, but of a different sort. Longitudinal aberration produces 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 minimized by using a small aperture. Transverse chromatic aberration is most pronounced in telephoto lenses. However, lenses can be designed in many ways. So called achromatic lenses have minimal chromatic aberration and are very popular. So called superacromatic and apochromatic lenses virtually eliminate color errors, but they are not common. Chromatic aberration can be observed 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.
Chromatic aberration can be corrected with software. 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 can be 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. We saw that chromatic aberration is a lens error; purple fringes on the other hand are sensor 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 original 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.
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