Jack Bowling asked:

Perhaps someone with biological info on the optical
characteristics of owl vision could inform us further.

Here is some information from a volume I have used a lot. Later today I'll
add a passage from a fine monogram on Barn Owls. With all the rain/snow
outside this is the second best thing to birdwatching.

Owls of the World, Their Evolution Structure and Ecology; edited by John
Burton, published by Dutton 1973
pg. 34
Sight is of vital importance to all birds; this is reflected in the
relatively enormous size of the eyes, which take up a huge amount of space in
the head; in most birds the two eyes are separated only by a thin bony sheet
in the mid-line of the skull. The eyeballs of some large hawks and owls are
in fact, bigger than our own. Underlying this disproportionate size is a
basic design limitation in the structure of eyes. This limitation is imposed
by the light-sensitive cells of the retina, the rear part of the eye on which
the image is thrown. The resolution of the eye - that is, its capacity to
discern detail - depends on the number of retinal cells over which the image
is spread, a greater number giving more detailed perception. Since the size
of these cells is roughly constant, it follows that a larger eye will
generally provide higher resolution than a smaller one of the same basic
shape. This limitation is a particularly acute problem for owls, since their
eyes are also modified for extreme sensitivity under poor lighting conditions
and, in many respects, adaptations for high sensitivity run counter to those
for high resolution.
The light-sensitive cells of the retina are of two types, named from their
shape, rods and cones. The rods are more sensitive to low light, but they
achieve this sensitivity partly by functioning in small groups, each group
stimulating only a single nerve cell, whereas cones generally each have their
own nerve cell to conduct their response to the brain. The result of this is
that image details falling on a group of rods are merged and lost, whereas,
had they fallen on cones, they might have been separately perceived.
A retina with many rods, as found in the eyes of owls and other nocturnal
animals, has thus sacrificed some of its capacity for resolution. Many
nocturnal animals also possess a reflecting layer - the tapetum - inside the
eye, to use more fully the light which enters, but the presence of such a
layer in the eyes of owls has yet to be conclusively shown.
A further adaptation to low light intensities is to bring the lens and
retina close together, so that the image itself is less dispersed and
consequently brighter. In consequence it is also smaller, and hence spread
over fewer retinal cells, thus further worsening the capacity for resolution.
Enlarging the eyes to the maximum has helped owls to overcome these
disadvantages, but to do this they have had to accept a sacrifice in field of
view, so that, although the image is spread over an adequate area of retina
for purposes of resolution, it covers a smaller proportion of the owls
surroundings than in other birds. This has led to the evolution of the
tubular eye, with a relatively abbreviated retina, and a huge, highly
convex lens. As in all birds, they eyeball is protected by a ring of small
bony plates (scleral ossicles), and in owls these form a long bony tube which
is one of the most striking features of a prepared skull specimen. In many
hawks the eye is superficially similar, but lengthening has taken place
without any proportionate enlargement of the lens, or reduction of retinal
area. The object in their case is to spread the image over as many retinal
cells as possible, to give maximum resolution.


Jim Rosso
Issaquah\
206-392-8440