Definition: Design of optical system in which four separate wavelengths are brought to a common focus. * Three wavelengths brought to a common focus (apochromat) is generally sufficient for very high quality images in the visible spectrum: the fourth is generally used for correction either into the ultra-violet or infra-red, enabling the lenses e.g. to be focused in visible light for recording in the extended wavelength. * They are also damned fine optics for normal photography.

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