Chromatic aberration.
A common lens flaw which results in loss of sharpness and also in colour fringing - white light having coloured halos (commonly green or purple) around the edges.
It’s caused by different wavelengths of light (ie: different colours) having slightly different focal points and magnifications. Think of a prism breaking white light into a colourful spectrum of light - a phenomenon known as dispersion. This can be considered to be an extreme example of the way refractive materials alter light in different ways according to the wavelength.
Typically, special glass with a low refractive index (ED glass for Nikon and UD glass and fluorite for Canon) is used to correct for chromatic aberration, particularly on telephoto and retrofocus wide-angle lenses, both of which have very assymetrical lens designs.
Technically there are two forms of chromatic aberration - longitudinal (or axial) and lateral (or transverse (TCA) or chromatic difference of magnification). The former is the problem of different wavelengths of light failing to focus on the same plane and can be minimized by stopping down the lens. The latter results in different wavelengths of light being shifted laterally across the image plane because of changes in image magnification in peripheral areas of the lens, which results in noticeable colour fringes or lateral colour. Lateral chromatic aberration is very common on inexpensive lenses and cannot be reduced by reducing the lens aperture.
cf. aberration, achromatic, apochromatic, dispersion, ED, fluorite, prism, refractive index, retrofocus, spectrum, telephoto lens, UD.
Entry last updated 2002-05-09. Term 211 of 1487.
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