Chromatic aberration is a phenomenon of lenses, whereby light of some colours is focused more strongly than others. In a colour image, this leads to colour fringes, often most visible at the edges of objects. In a black-and-white image this is rendered as simple blurring in the same places.
It is one objective of lens design to reduce chromatic aberration, by matching lens elements so that the chromatism of one element cancels out that of the others. The names of some lenses (and classes of lens) of the mid-20th century refer to their being well-corrected in this way. An 'achromat' is a lens designed so that the focus is exactly the same at two different wavelengths of light, and an 'apochromat' is corrected at three wavelengths. More corrections are possible; Jacobson refers to superachromats corrected at four wavelengths, and states that one of the corrected wavelengths may be in the infra-red, so visual focusing can be used for infra-red film.
A related term, encountered in old discussions of lenses, is chemical focus. This is the phenomenon whereby the image captured on a plate or film shows different parts of the scene in focus than were seen on the ground-glass, because early plates were sensitive only to the blue end of the spectrum, and early lenses suffered from chromatic aberration.
- Jacobson, Ralph E. (Ed.), 1978, The Manual of Photography 7th Edition. Focal Press, London. ISBN 0 240 50957 9. Pp 107-9.