Pupil shapes and lens optics in the eyes of terrestrial vertebrates

Animal eyes that are primarily used under low-light conditions usually have optical systems of short depth of focus, such that chromatic defocus may lead to considerable blurring of the images. In some vertebrates, the problem is solved by multifocal lenses having concentric zones of different focal lengths, each of which focuses a different relevant spectral range onto the retina. A partially constricted circular pupil would shade the peripheral zones of the lens, leading to the loss of well-focused images at relevant wavelengths. The slit pupil, however, allows for use of the full diameter of the lens even in bright light. We studied species of terrestrial vertebrates from a variety of phylogenetic groups to establish how widespread multifocal lenses are and how pupil shapes are adapted to the optical systems. We found that multifocal lenses are common from amphibians to mammals, including primates. Slit pupils were only present in animals having multifocal optical systems. Among the felids, small species have multifocal lenses and slit pupils, while large species have monofocal lenses and round pupils. The Eurasian lynx, a cat of intermediate size, has an intermediate eye design. The functional significance of the absence of multifocal optical systems in large felids remains mysterious, because such systems are present in other large-eyed terrestrial vertebrates. Multifocal optical systems in nocturnal prosimians suggest that those animals have colour vision despite being described as cone monochromats.