achromatic lens

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achromatic lens

(ak-rŏ-mat -ik) (achromat) A two-element lens – a doublet – that greatly reduces chromatic aberration in an optical system. The components, one converging and the other diverging in action, are of different types of glass (i.e. they have different refractive indices); the combination focuses two selected colors, say red and blue, at a common image plane with a small spread in focal length for other colors. The difference in optical power (reciprocal of focal length) for the two colors in one element must cancel that in the other element. By a suitable choice of glasses and surface curvatures, the doublet can be aplanatic as well as achromatic, so that three major aberrations are minimized (see aplanatic system).

Residual color effects in an achromat can be further reduced by using a compound lens of three or more elements – an apochromatic lens; each element has an appropriate shape and dispersive power so that three or more colors can be focused in the same image plane.

achromatic lens

[¦a·krə¦mad·ik ′lenz]
(optics)
A combination of two or more lenses having a focal length that is the same for two quite different wavelengths, thereby removing a major portion of chromatic aberration. Also known as achromat.
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While the human eye can't always tell the difference in image quality between fully multicoated premium crown and flint achromatic lenses and ED or fluorite crystal lenses, film can.
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But since CCDs are relatively small this tolerance can be relaxed, and many focal reducers suitable for digital imaging can be made from simple achromatic lenses such as those scavenged from a old pair of binoculars.