Chromatic aberration is caused by the dispersion of the lens
material, the variation of its refractive index n with the wavelength
Since the focal length f of a lens is dependent on n, it follows that
different wavelengths of light will be focused to different positions.
Chromatic aberration of a lens is seen as fringes of color around the
image, because all colors cannot be focused at a single common position
on the optical axis. However, there exists a point, called circle of
least confusion, where this effect can be minimized.
Chromatic aberration can be further minimised by using an achromatic
doublet or achromat in which two materials with differing
dispersion are bonded together to form a single lens. This reduces the
chromatic aberration over a certain range of wavelengths,
though it does not produce perfect correction.
Also, special glasses have been developed, notably containing the mineral
called fluorspar, which have such a low dispersion that
using two lenses already give a very good correction.
Optical engineers can design lenses that bring several colors to a
focus at the same distance from the lens. Such lenses
can be apochromatic, and can give sharper images than uncorrected optics.
However, it requires
extra pieces of glass in the optical path, and additional
kinds of glass. This makes apochromatic lenses heavier and more expensive
An apochromatic lens is often
called an apochromat, or apo, for short.