equiconcave

An equiconcave lens has two inward-curving surfaces with identical radii of curvature. Both sides bow toward the center of the lens, creating a shape like two bowls placed face-to-face. Because the curves match, the lens behaves symmetrically along its optical axis, and the focal length depends only on the refractive index of the glass and the magnitude of the shared radius.

Equiconcave lenses are diverging elements, meaning they spread light rays outward. They are weaker divergers than plano-concave lenses of the same radius, because the symmetrical curvature distributes the optical power more evenly. A typical equiconcave lens made from borosilicate crown glass with a 50 mm radius of curvature produces a focal length around 100 mm.

Equiconcave lenses appear in compound optical systems where controlled divergence is required without introducing strong spherical aberration. They are also used in eyepieces, as negative components in afocal (telescopic) systems, and as corrective elements in optical benches. When coated with anti-reflection materials on both surfaces, they integrate cleanly into multi-element assemblies because the symmetry simplifies calculation of reflection losses.

The main manufacturing constraint is centering: both surfaces must be ground and polished to the same radius with tight tolerances, or the lens will deviate from the optical axis. Surface irregularity on either face degrades performance quickly, since equiconcave lenses are often thin and thus sensitive to form errors. Quality control usually includes interferometric testing to verify radius match and surface quality.

The term distinguishes this lens from biconvex (both surfaces bulging outward), plano-concave (one flat, one concave), or meniscus variants. In optical design shorthand, equiconcave lenses are often notated as symmetric negative elements and appear in tables as simply '−R, −R', where R is the radius value.