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An achromatic doublet

A small brass camera lens sits at an angle on a flat white background. The front lens element is visible, facing up and to the left. Some text is visible around the front lens element: "[…] Jena Nr 289072 Tessar 1 : 4, 5 […]". — An old Carl Zeiss Tessar camera lens with four elements, comprising two doublets. The front doublet is air-gapped and divergent; the rear doublet is glued and convergent. This arrangement was better at correcting spherical and chromatic aberrations and astigmatism than previous lens designs.

In optics, a doublet is a type of lens made up of two simple lenses paired together. Such an arrangement allows more optical surfaces, thicknesses, and formulations, especially as the space between lenses may be considered an "element". With additional degrees of freedom, optical designers have more latitude to correct more optical aberrations more thoroughly.