![]() When a circular pupil constricts, the peripheral zones of a multifocal lens are no longer involved in image formation, thus preventing the suggested improvement in image quality. They argued that the multifocal arrangement is useful because it allows reasonable image sharpness across a range of wavelengths at the expense of some contrast: “dividing the lens into three zones of equal aperture areas and focusing a specific wavelength improves the functionality of the lens in comparison to a monofocal lens”. This lens has concentric zones of different focal lengths, with each zone focusing a different wavelength band onto the retina. Kröger and colleagues ( 9, 10) proposed that some animal eyes minimize blur due to chromatic aberration with a multifocal lens. This chromatic aberration produces noteworthy blur in images containing a wide range of wavelengths. Simple lenses focus different wavelengths at different distances: for example, blue at nearer distance than red. Control of retinal illumination in different light environments When the relative-risk ratio is greater than 1, the directional change in the independent variable (foraging or activity) was associated with a greater probability of having the specified pupil shape than a horizontal pupil. Foraging mode proceeded from herbivorous prey to active predator to ambush predator. Activity time proceeded from diurnal to polyphasic to nocturnal. Relative-risk ratios were computed for having a circular, subcircular, or vertical-slit pupil relative to having a horizontal pupil as a function of foraging mode or diel activity. Multinomial logistic regression tests were conducted with foraging mode, activity time, and pupil shape as factors and genus as a covariate. ( C) Results of statistical tests on the relationship between foraging, activity, and pupil shape. The dots in each bin have been randomly offset to avoid overlap. Colors represent diel activity: yellow, red, and blue for diurnal, polyphasic, and nocturnal, respectively. The axes are pupil shape and foraging mode (herbivorous prey, active predator, or ambush predator). ( B) Pupil shape as a function of foraging mode and diel activity. From top to bottom: vertical-slit pupil of the domestic cat, vertically elongated (subcircular) pupil of the lynx, circular pupil of man, and horizontal pupil of the domestic sheep. Horizontally elongated pupils create sharp images of horizontal contours ahead and behind, creating a horizontally panoramic view that facilitates detection of predators from various directions and forward locomotion across uneven terrain. This is advantageous for ambush predators to use stereopsis to estimate distances of vertical contours and defocus blur to estimate distances of horizontal contours. Vertically elongated pupils create astigmatic depth of field such that images of vertical contours nearer or farther than the distance to which the eye is focused are sharp, whereas images of horizontal contours at different distances are blurred. Species with horizontally elongated pupils are very likely to be prey and to have laterally placed eyes. Species with vertically elongated pupils are very likely to be ambush predators and active day and night. There is a striking correlation between terrestrial species’ pupil shape and ecological niche (that is, foraging mode and time of day they are active).
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