For other uses of this term, see Meniscus.
A meniscus is a lens formed by a concave and a convex surface. It has a positive or negative focal length. Corrects the spherical aberration of a convex plane lens.
The optimal meniscal orientation for most applications is achieved with the smallest radius of curvature as the first surface. The meniscus is used in combination with other components, being able to increase or reduce the focal of an assembly without significantly increasing axial aberrations.
wiki
