Thus, slit pupils allow for use of the entire diameter of the eye and each zone of differential focus even when high intensity light is encountered. If the animal had circular pupils then if the animal experienced intense light during the day when the pupil constricts it would cut out many of the zones of differential focus creating a very blurry image. Due to multifocal lenses and the nocturnal mammals eye being highly sensitive to bright light circular pupils would not work.
These concentric zones focus a specific spectral range onto the retina allowing for better resolution of the image. However, many animals have adapted by having multifocal lenses with concentric zones of differential focal lengths. This chromatic defocus can cause blurry images. This chromatic defocus is due to light hitting a medium and differing wavelengths travelling at different speeds causing chromatic defocus or chromatic abberation. This is due to the pupil being so large in comparison to the focal length. ĭue to these very large pupils nocturnal mammals have a small depth of field. Light can further be reduced by having a vertical slit and partially closing the eyelids. The way that nocturnal or diurnal mammals accommodate such a wide range of light intensities is through having a slit pupil. ĭue to this increased sensitivity the animal needs to protect its eyes from the high intensity light that it could possibly experience in the day. The way that nocturnal mammals adjust for this is by increase “quanta” or light capture by having large pupils and short focal lengths. Additionally, at lower light levels it is harder to distinguish edges and borders and thus shapes because there is a decrease in contrast in low light conditions. Thus, the large pupil helps to receive as much light as possible in low light conditions. Having a larger eye and larger pupil allows a nocturnal mammal to receive more incoming light thus gather more photons due to the larger surface area exposed. Nocturnal mammals tend to have larger eyes as well as larger pupils that can open widely in low light.
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