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Chromatic aberration is an important photographic phenomena to grasp. It might seem complicated, but it is easy enough. 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 lens does not focus the different colors on the same sensor plane. 2. The different colors produce images of different size. Here we will take a deeper look at what chromatic aberration is and how to avoid or solve it.
The first thing to understand 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. The problem is that the different wavelengths of light have different refractive indexes. For example blue might leave the lens at 79 degrees while red could leave at 81 degrees. This produces what is known as longitudinal chromatic aberration where you see 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 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 creates 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.
Both types of chromatic aberration produce color fringes, but of a different sort. Longitudinal aberration creates magenta fringes around objects and is distributed uniformly throughout the image. Transverse aberration spreads radially, meaning it is absent in the center and most pronounced at the 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. 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. 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.
Chromatic aberration can be fixed 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 is sharp. Transverse chromatic aberration can be 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 real color is usually eradicated. 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.
Source: Chromatic aberration 1 and Chromatic aberration 2<