The deep ocean is a dim place. The amount of sunlight that reaches a depth of 300 m is roughly 1/10 000 of that at the surface. Squid have adapted to those low-light aquatic conditions by evolving eyes with lenses that are nearly spherical in shape. A spherical lens has the shortest possible focal length for a given lens radius. It thus optimizes the size of the eye opening relative to the size of the image it makes on the retina. Unlike the oblong human lens, which changes shape to change focal length, the squid lens has a fixed focal length. Squid focus by moving the lens. But a spherical lens can suffer spherical aberration, and the greater the focusing power, the more serious the aberration.
In 1854 James Clerk Maxwell showed theoretically that a spherical lens could be made aberration free if its refractive index varied parabolically as a...
