What’s achromatic lens ?

Achromatic lens is designed for limiting the chromatic effect and aberration. The lens is designed to bring red and blue wavelengths to focus on the same plane. Normally, achromatic lenses compose two individual singlet lenses which are made with different amounts of dispersion. Normally, one lens is negative with flint glass like F4 or ZF2, and another lens is positive lens with glasses like BK7, BK7 has lower dispersion. The two lenses are cemented to each other by optical glue.

The positive power of the crown lens is not quite equaled by the negative power of the flint lens. Together they form a weak positive lens that will bring two different wavelengths of light to a common focus. Negative achromatic lenses in which the negative-power element predominates are also made.