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 wellfocused
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.