The visual performance of barn owls is hyperacute in the binocular and monocular domain, both demonstrated in behavioural acuity tasks, i.e. stereo and vernier acuity. We were interested to find out to what extent spatial resolution in this bird is limited by the optical properties of their eyes. For that purpose eight eyes from 4 adult American barn owls (Tyto alba pratincola) were studied. Ocular aberrations are measured to the 6th – order Zernike polynomials using a Tscherning-type wavefront aberrometer. During the measurement owls were awake and manually restrained, their pupils were normal. No drugs were applied. Owls blinked naturally.

The ocular wavefront, described by the weighted sum of Zernike polynomials, is an indicator for what happens to the light travelling into the eye when reaching the photo-sensitive layer, the retina. Its impulse response is also known as the point-spread function (PSF) and is the theoretical image on the retina that would be formed by a point light source. The fourier transformation of the PSF is the modulation-transfer function (MTF), which describes how well different spatial frequencies are transferred by the optics of the eye.

Barn owl in front of the aberrometer

We found that higher order aberrations were very low in all eyes - about 3 times lower than those of human eyes, allowing a retinal image formation of excellent quality. This is an interesting finding, because it is known that barn owl photoreceptor cell density allows no better resolution than about 8 cyc/deg. The question arising from this is why the owl needs this superior optical quality, considering its neural sample pattern unable to resolve fine details. One answer might be, that given the excellent optics which raise the MTF over all spatial frequencies helps the owl avoid hitting low frequency objects while flying in the dark. Also, items of prey, otherwise invisible at low light conditions might become visible, due to an elevated modulation transfer and, thus, elevated contrast.