achromatic lens

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.