These lenses consist of two optical elements, usually of crown and flint glass types, cemented together to form an achromatic doublet. Most often, the crown lens is a biconvex positive lens. Due to the difference in refractive indices, chromatic aberration with respect to two selected wavelengths has been corrected. The lenses have been corrected for on-axis spherical aberration as well. Achromatic doublets are used as telescope objectives, eye loupes, magnifying glasses and eyepieces. Achromatic doublets have also been used to focus and manipulate laser beams because their image quality is superior to single lenses.