spherical lensed eye

A spherical lensed eye contains a single lens with a uniform refractive index throughout. Light is bent (refracted) as it passes through the curved lens surfaces, converging rays onto the photoreceptor layer. Because a homogeneous sphere produces spherical aberration (peripheral rays focus at a slightly different point than central rays), many animals with this eye type compensate through the density gradient of their photoreceptors or by the positioning of the retina.

Spherical lensed eyes are common among aquatic invertebrates and fish. Many cephalopod molluscs (squid, cuttlefish), various crustaceans, and bony fish possess this type. The near-perfect spherical lens of fish eyes is particularly well-adapted for underwater vision, where the high refractive index needed (water reduces the air-glass refractive contrast) makes a sphere the most efficient geometry.

A spherical lens with uniform refractive index suffers from spherical aberration, reducing image sharpness. Many animals (including fish and cephalopods) instead use gradient-index (GRIN) lenses, where the refractive index is highest at the center and decreases toward the periphery — this corrects for aberration and produces a sharper image. Such GRIN lenses can appear spherical externally while functioning optically superior to a uniform sphere.