Apochromatic is a description of a photographic or other lens having a high degree of color correction. Chromatic aberration is the phenomenon of different colors focusing at different distances from a lens (see the article for an excellent illustration). In photography, it leads to fuzzy images, and to color fringes at high-contrast edges, like an edge between black and white. Astronomers face similar problems, particularly with telescopes that use lenses rather than mirrors. General purpose photographic lenses are corrected to bring two of three primary colors into critical focus in the same plane. Apochromatic lenses are generally designed to bring all three Red/Blue/Green. Graphic Arts process (copy) cameras generally use APO lenses for this reason. Classically designed process or APO view camera lenses generally have a maximum aperture limited to F9. Recently high speed APO lenses have been produced for medium format, digital and 35mm cameras.

Optical engineers can design lenses that bring several colors to a focus at the same distance from the lens. Such lenses can be apochromatic, and can give sharper images than uncorrected optics. However, it requires extra pieces of glass in the optical path, and additional kinds of glass. This makes apochromatic lenses heavier and more expensive than their uncorrected counterparts.

An apochromatic lens is often called an apochromat, or apo, for short.

Dpreview's Chromatic Aberration related page