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Doublet is a lens design comprised of two elements grouped together. Sometimes the two elements are cemented together, and other times they are separated by an air gap. Doublets are primarily used to correct chromatic aberration, spherical aberration, and coma. An achromatic doublet consists of a positive low-index crown glass element cemented to a negative high-index flint glass element. The elements are chosen so as to cancel chromatic aberration at two well separated wavelengths; usually in the blue and red region of the spectrum. Focal length is constant at those two wavelengths and focal length shifts are virtually eliminated across the visible wavelengths. By minimizing optical aberrations, achromatic doublets produce sharper, clearer images with accurate color reproduction.