The optical performance of compound lenses, two or more single elements designed to work together, is superior to that of single lens elements working alone. Achromatic doublets and triplets are compound lenses composed of two and three singlets. Designers often abbreviate their reference to achromatic doublets and triplets by simply calling them achromats.

The origin of the technical term "achromatic" is in the color-corrected performance of compound lenses. They are designed to outperform singlets by all measures, but designers give special attention to the reduction of color effects in their images. "Achromatic" literally means "without color," and in lens design, the term refers to minimal difference of focal length between two specific wavelengths.

Achromats are the simplest of the multi-element or compound lens designs. Aberrations inherent in a single lens element limit imaging performance. By carefully combining complementary singlets and assembling them into a compound lens, designers can reduce the net aberration of the image. Aberrations induced by one singlet can be cancelled by the opposing aberrations of other singlets.

Achromatic doublets generally demonstrate these principles: one element is made of crown glass (low refractive index and low dispersion); the other is made of flint glass (high refractive index and high dispersion).

The ultimate quality of an image often can be related to the number of elements within the lens. A rule of thumb states, "The more elements, the better the quality." This rule is usually valid because more elements give the designer more degrees of freedom to control aberrations in the image.

Achromatic doublets and triplets deliver about the same image quality; the advantage of the triplet over the doublet lies in its ability to work at lower f-numbers (higher speed) and larger fields of view.

Achromats, a class of lenses whose image quality is superior to that of singlets, include cemented doublets, air-spaced doublets and cemented triplets. Improved performance is achieved with the two different kinds of glass. Multiple elements provide added degrees of freedom for the designer to balance aberrations near the optical axis. Achromats are useful in applications requiring a lens with a narrow field of view and large to moderately small f-numbers.

The difference between achromatic triplets and doublets is small. Sometimes the slightly lower f-number or slightly larger field of view of a cemented triplet makes it a better candidate for an application than a cemented doublet. In addition, the symmetrical construction of most cemented triplets enhances performance for 1:1 imaging.