The Optics of Animal Eyes

Eyes with well-developed optical systems evolved many times at the end of the Cambrian period. 500 million years ago. There are now about ten optically distinct mechanisms. These include pinholes, lenses of both multi-element and inhomogeneous construction, aspheric surfaces, concave mirrors, apposition compound eyes that employ a variety of lens types, and three kinds of superposition eye that utilize lenses, mirrors, or both. Because the number of physical solutions to the problem of forming an image is finite, convergent evolution has been very common. The best example is the inhomogeneous Matthiessen lens, which has evolved independently in the vertebrates, several times in the molluscs and annelids, and once in the crustaceans. Similar cases of convergence can also be found among compound eyes.