Achromats are objectives corrected for chromatic aberration in the range of red and blue light (656nm and 486nm). Additionally they have spherical correction in the range of yellow-green light at 540nm. The chromatic correction is achieved by the combination of at least two different lens elements, one consisting of flint and the other of crown glass. The lower dispersion of crown glass compensates for the chromatic aberration of flint glass. When working at low magnifications (below ?40x) Achromats give satisfying results in white light. The major advantage of achromatic objectives is their cost effectiveness in relation to higher quality lenses with even better aberration correction (Fluorites, Apochromats).