An achromatic is a lens with two or three closely-spaced, often
cemented, elements designed to produce images largely free from false color.
It has at least one element of flint glass and another of crown glass – the dispersion of the latter compensating
for the chromatic aberration of the former. While achromatic lenses bring two primary colors (red and
blue) into focus at the same point, they leave some uncorrected chromatic
aberration at wavelengths in between. This variation is negligible for most
purposes, and a single focal-length value is applied to the entire visible
spectrum. Aberration can be further reduced by using an apochromatic lens.
AND OPTICAL PHENOMENA
EQUIPMENT AND TECHNIQUES