|landscape lens and prisms illustrating how the glass combination works|
The achromatic lens is a simple solution to reduce chromatic aberration. It is a lens consisting of one group of two or three lens elements of different glasses, which may be cemented, and designed so that the chromatism of one element is offset by that of the other. An old landscape lens may consist of such a group, i.e. a biconvex crown glass element and a biconcave flint glass element. Other achromatic lenses use other variants of concave flint and convex crown glass. Many box cameras have a meniscus achromat. In addition to the correction of chromatic aberration, Greenleaf (1950) states that compared with the simple meniscus lens, a skillfully designed meniscus achromatic lens exhibits also reduced spherical aberration and coma.
Two such achromats, arranged symmetrically, form a Rapid Rectilinear (or Rapid Aplanat), which corrects more lens aberrations.
- Greenleaf, Allen (1950) Photographic Optics. MacMillan, New York. pp65-6.