Above and below the water

Abstract

The invention of lenses represented a considerable advance in image perception. For instance, in octopi with only pinhole camera eyes, the image projected onto a retina is noticeably darker than in octopuses with lens eyes. Organisms that dwell in water mostly exhibit spherical lenses, yet still manage to attain a sharp image on the retina due to a decreasing refractive index from the lens center towards its outer regions (also called an inhomogenous lens). This reduces refractive distortions considerably and mostly shortens the overlong focal distance, enabling a clear picture on the retina.