Chromatic aberration

The concept of chromatic aberration covers several different optical phenomena (lateral chromatic aberration, longitudinal chromatic aberration) which all result in undesirable coloured fringes around the features of an image. This is caused by the fact that in a converging lens, for example, the short wavelength rays (violet/blue) converge nearer to the lens than the longer wavelengths (red): diffraction does not occur at the same angle for all colours.

The phenomenon shows up very clearly around areas of the image where there is a transition from high contrast to relatively neutral parts of the image.