Chromatic aberration
Chromatic aberration happens because the refractive index of glass is different for different colours. This causes light rays of different frequencies to end up in different places on the sensor even if they have the same source. The result is colour fringing and blurry pictures.
The cause and cure for chromatic aberration has been know for a very long time, some of the early computers made by Konrad Zuse were used to calculate the properties of compound lenses. Unfortunately good optics is expensive so we still have the problem to some degree.
Combining different types of glass in a compound lens is a common method of correction. There are also special types of glass with very low levels of optical dispersion that is commonly used. High quality optical low dispersion glass is very expensive so it is usual to have only one or two such lens elements in a photographic lens that may contain 10-15 lens elements in total.
The designer of the lens may decide to correct the errors to different degrees so chromatic aberration may show itself in many different ways. For example the eye and brain are more sensitive to errors in red and green than in blue so a cheap lens may leave the blue part of the spectrum uncorrected since it causes the least visible error in most pictures.
It is possible to correct affected pictures to a certain degree with computer software. The limit is set by the way a camera works. Each colour filter passes a broad range of light frequencies and it is not possible to know the exact frequency if a light ray after it has passed the colour filter. The ray is classified either as red, green or blue, this coarse measurement of frequency makes it impossible to accurately calculate the correct refraction.
