BK7メニスカスレンズ

Meniscus lenses can be either positive or negative, depending on the relative curvatures of the two surfaces. A negative meniscus lens is thinner at the centre than periphery. Conversely, a positive meniscus lens is thicker at the centre than periphery.

A thin lens with two surfaces of equal curvature has zero optical power, which means it does neither converge nor diverge light. All real lenses have thickness, because of its curvature, it affects the optical power. To achieve zero optical power, a meniscus lens must have slightly unequal curvatures to account for the effect of the lens thickness.