Chromatic aberration may sound difficult, but it is in fact quite simple. It is created by the camera lenses and is seen as magenta and blue-green fringes in the photos. There are two kinds of chromatic aberration: 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.
First we have to understand what refractive index 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). Chromatic aberration arises because the different colors of light have different refractive indexes. For example blue could leave the lens at 79 degrees while red could 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.
When light does not hit the lens at 90 degrees, but from an oblique angle, you begin to get transverse chromatic aberration.. 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. 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 distributed uniformly throughout the image. Transverse aberration spreads radially, meaning it is absent in the centre and most pronounced at 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. There are numerous lens designs. So called achromatic lenses have minimal chromatic aberration and are very popular. Superacromatic and apochromatic lenses virtually eliminate color errors, but they are not common. Chromatic aberration can be seen on film, but is most visible on digital images. 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. By sharpening the red and blue channels, one can somewhat correct longitudinal chromatic aberration; 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 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. 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 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.
First we have to understand what refractive index 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). Chromatic aberration arises because the different colors of light have different refractive indexes. For example blue could leave the lens at 79 degrees while red could 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.
When light does not hit the lens at 90 degrees, but from an oblique angle, you begin to get transverse chromatic aberration.. 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. 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 distributed uniformly throughout the image. Transverse aberration spreads radially, meaning it is absent in the centre and most pronounced at 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. There are numerous lens designs. So called achromatic lenses have minimal chromatic aberration and are very popular. Superacromatic and apochromatic lenses virtually eliminate color errors, but they are not common. Chromatic aberration can be seen on film, but is most visible on digital images. 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. By sharpening the red and blue channels, one can somewhat correct longitudinal chromatic aberration; 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 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. 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 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.