Definition: An optical system in which chromatic aberration has been corrected to the extent that three colours are brought to a common focus. * An optical system lens in which the secondary spectrum has been reduced. * This requires the use of specialised glass, with low or anormalous dispersion and uses at least three components of different kinds of glass. * In photography, usually applied only to longer focal length lenses (180mm and above). * Also called an apochromatic lens.
Once were uncorrected
Apochromatism in long focal length was once a dream of spotty technicians but is now commonplace amongst the top-grade long toms from the likes of Canon, Nikon, Zeiss and so on. It's been made possible not just by the invention of glasses with unusual dispersion properties but also the improvements in multi-coating that have made it possible to use many elements without losing quality.

Related Terms: chromatic aberration

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