Shape and focal properties of the cephalopod ocular lens

The shape and focal properties (spherical aberration) of lenses from the eyes of three cephalopod species, the cuttlefish Rosia pacifica, the squid Loligo opalescens, and the octopus Octopus dofleini, were measured and compared with descriptions of teleost lenses. The method used involved photographing the refractive effect of lenses in solution on an array of fine parallel helium–neon laser beams. Though the cephalopod lenses approach the sphericity characteristic of teleost lenses, the equatorial lens diameter is somewhat greater (2.3–10.5% in Loligo) than the axial diameter. Spherical aberration of the lens is largely neutralized, indicating the existence of a gradient refractive index. However, the lenses show some non-monotonic variation in focal length with distance from the centre of the lens. The ability of the cephalopods to control pupil size may minimize the need to control spherical aberration.