Falcon visual sensitivity to grating contrast

Abstract

The eye of a small falcon, the kestrel, is about half the size of the human eye and, if similar in all other respects, its visual acuity would also be expected to be about half that of the human. Here we report that the cut-off frequency (a measure of visual acuity) is twice the expected value and is the same for kestrel and human eyes. This remarkable visual performance is explained by anatomical features of the bird eye and the spherical pit of the deep fovea acting as the diverging element of a telephoto lens to magnify the image 1,2. In the low-spatial frequency range, however, falcon contrast sensitivity is much less than that of humans for stationary patterns. © 1982 Nature Publishing Group.