Telephoto lens system of falconiform eyes

Abstract

VISUAL ACUITY in falconiform birds has been shown to be higher than man. For instance, vultures with eyes similar in size to those of man have a grating detectability of about twice the spatial frequency of man¹. This is consistent with measured image quality in an eagle eye similar in size to the human eye². Recently a falcon (Falco sparverius) has been shown to have a grating detectability 2.6 greater than that of man³. Although the image quality of these falconiform eyes is at least twice as good as man, their minimum intercone spacing is only slightly less than in the human retina. Unless the ratio of focal length to the axial eye length of these birds greatly exceeds that in man, falconiform eyes, unlike man, would be unable to resolve their best retinal image quality. We show here that the presence of a spherical depression in the deep fovea of falconiforms may act like the negative lens component in a telephoto lens system or opera glass. The focal length of the birds' dioptrics can then in theory exceed the axial length of the eye, thus providing the relatively large images necessary for more complete image reconstruction (high resolving power) in a localised region of the retina.