CA - Chromatic Aberration
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Chromatic Aberration -CA for short- also known as Achromatism in Optics and as "Fringing" colloquially is the condition of a lens which cannot focus all colours at the same plane.
It is caused by the differences in refraction of the coloured rays of the spectrum. It is "axial" when light rays pass through a lens cause the lens to focus at different points, depending on the light wavelength. It is "lateral" or "transverse" when the magnification varies depending on the light wavelength. It creates blur.
Chromatic aberration shows up as "fringes" of colour along edges that separate dark and bright parts of the image, because each colour in the optical spectrum cannot be focused at a single common point on the optical axis. It should be noted that "fringing" may also occur as a result of lens flare and not only because of chromatic aberration.
Nikon optical designers and Nikon glass specialists, developed ED (Extra-low Dispersion) glass, used in selected telephoto and telephoto zoom lenses, providing superior superior sharpness and colour correction by effectively minimising chromatic aberration to such a minimal degree that it is remarkable in telephoto lenses.
