An achromatic assembly is a group of lenses designed to compensate for various unwanted optical effects. Achromatic assemblies are used in most modern cine lenses. They are also used in the +1 and +2 ARRI/Zeiss Master Diopters.

A single lens bends different colors at different angles.

An achromatic lens assembly ensures that all colors are focused at the same point.

Achromatic assemblies are particularly good at compensating for chromatic aberration, an optical effect that can be seen as color fringing in the image, usually at high contrast edges. They also improve spherical aberration, an effect where a white point is projected not as a point but as an out of focus blurb.

When a beam of light passes through a single lens, the different colors are bend at different angles based on their wavelengths (called dispersion), and the lens will focus the different colors at different points. The result is chromatic aberration, an unwanted color fringing that will lower the lens' resolution and that is also problematic in special effects matting and blue and green screen work.

In a two lens achromatic assembly, one lens is compensating the dispersion introduced by the other. For such purpose, one lens is usually made in a crown glass with low dispersion, while the other is made of a flint glass with high dispersion. The crown lens performs the desired optical effect (for example magnifying an image) and introduces some dispersion, and the flint lens aims at balancing this dispersion while having the least possible optical influence itself.