The eye of the humboldt penguin, Spheniscus humboldti : visual fields and schematic optics

Abstract

Construction of a schematic eye indicates that the eye of Spheniscus humboldti is aquatic in design. The lens has a power of 100 dioptres (D) while (in air) the cornea has a power of 29 D. In air, the eye is myopic (approximately 28 D) but in water it is emmetropic. Minimum pupil size would seem insufficient to allow the pupil to function as a stenopaic aperture and increase depth of focus sufficiently to overcome the eye’s aerial myopia. Entry into water reduces maxim um image brightness by approximately three times. In air, the maximum width of the retinal binocular field is 45° and this occurs approximately 10° above the line of the bill. The bill intrudes into the retinal field and binocular field width in the plane containing the bill and the optic axes is 28°. The vertical extent of the binocular field is 125°. In the plane containing the optic axes the cyclopean field equals 282° and the optic axes diverge by 116°. In this plane the mean uniocular field is 155° with the temporal hemifield approximately 11° larger than the nasal hemifield. Entry into water reduces the widths of the visual fields such that maximum binocular field width is only 17° and the vertical extent is reduced to about 80°. Binocular vision is lost in the plane of the bill, and the uniocular retinal field is reduced by 32° and the cyclopean field by 36°.