Chromatic aberration is an important photographic phenomena to grasp. It might sound difficult, but it is easy enough. It is seen in photos as magenta and blue-green fringes produced by the lenses. 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. In the following article we will look in depth at the phenomena of chromatic aberration and how to avoid or solve it.
The first thing to understand 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. 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 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. 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 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 creates magenta fringes around objects and is spread uniformly 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 minimized 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 so called "superachromatic" and "apochromatic" lenses, that almost eliminate chromatic aberration. 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.
Chromatic aberration can be fixed with software. 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 can be 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 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.
The first thing to understand 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. 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 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. 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 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 creates magenta fringes around objects and is spread uniformly 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 minimized 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 so called "superachromatic" and "apochromatic" lenses, that almost eliminate chromatic aberration. 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.
Chromatic aberration can be fixed with software. 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 can be 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 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.
No comments:
Post a Comment