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Abstract

We have examined the optical properties of the crescent-shaped pupils which are found in a variety of vertebrates including skates and rays. We found that this pupil shape preserves a small depth of field while limiting light flux to the retina. This pupil shape will also decrease the effects of lenticular spherical aberration, provide a larger visual field, provide a higher theoretical resolution limit, enhance contrast at high spatial frequencies, and provide information on the sign and degree of visual defocus relative to a reflective object. With a crescent-shaped pupil, reflected points of light (such as from a fish) will be imaged as ∪'s if the eye is focused in front of the object, as points if the eye is focused on the object and as ∩'s if the eye is focused behind the object. As the degree of ametropia increases relative to the object plane, the size of the ∪ or ∩ increases.

We have previously demonstrated that the multiple pupillary apertures which are characteristic of gekkos, a group of primarily nocturnal lizards, limit retinal illumination while maintaining a shallow depth of field (Murphy and Howland '86). With such a pupillary design, an object in the environment will form a single image on the retina only if it lies within the animal's plane of focus. Objects lying either in front of or behind the focal plane will form multiple images on the animal's retina. We propose that this principle is true for all species with multiple pupillary apertures, including some species of sharks, skates, and rays, some cetaceans, llamas, horses, and many artiodactyls. Additionally, similar to crescent-shaped pupils, this pupil shape provides a larger visual field than a circular pupil of identical surface area and may diminish the effects of lenticular spherical aberration.