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Chromatic aberration

Failure of a lens to focus all colors on the same point / From Wikipedia, the free encyclopedia

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In optics, chromatic aberration (CA), also called chromatic distortion and spherochromatism, is a failure of a lens to focus all colors to the same point.[1] It is caused by dispersion: the refractive index of the lens elements varies with the wavelength of light. The refractive index of most transparent materials decreases with increasing wavelength.[2] Since the focal length of a lens depends on the refractive index, this variation in refractive index affects focusing.[3] Chromatic aberration manifests itself as "fringes" of color along boundaries that separate dark and bright parts of the image.

Focal length of lens varies with the color of light
Photographic example showing high quality lens (top) compared to lower quality model exhibiting transverse chromatic aberration (seen as a blur and a rainbow edge in areas of contrast.)