Hawks, vultures and other raptors have great visual acuity and
telescopic vision. Their eyes are tubular in shape, which accounts for
this feature. But their eyes have a single lens. Optical refractive
telescopes have two or more lenses. Is it possible to construct a
single lens telescope, perhaps with a soft, flexible lens?

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Request for Question Clarification by revbrenda1st-ga on 03 Jan 2003 12:27 PST

Do you mean a "real" telescope such as you'd purchase at a store? I've
found information about how to gerry-rig some simple materials to get
a telescopic effect using just one lens, but it isn't the real deal.

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Clarification of Question by knowitall22-ga on 03 Jan 2003 13:10 PST

revbrenda1st:  Basically, I'm curious as to how birds can have
marvelous telescopic vision with one lens per eye. Nature is subtle
and miraculous in many ways, often beyond our comprehension. But
vision must obey laws of optics. Has any researcher analyzed bird
vision from a purely physical viewpoint by trying to duplicate it with
an apparatus? So yes, is it possible to build a "real" working
telescope (equivalent to a hawk's vision)? I suspect the answer is No.
Thanks,
knowitall22

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Request for Question Clarification by revbrenda1st-ga on 03 Jan 2003 15:04 PST

You said: "So yes, is it possible to build a "real" working
telescope (equivalent to a hawk's vision)? I suspect the answer is
No."

Well, knowitall22, according to this website, the first telescopes
were made with just one lens. It wasn't until the mid-1700s that
telescope makers improved the design by adding another lens to fix
"chromatic aberration." Therefore it is possible to build the thing.
As far as duplicating a hawk's vision, no matter how hard man may try,
he's never going to be able to achieve what nature does so easily.

http://www.space.com/scienceastronomy/astronomy/telescopes_III.html

Here's the address of the website I mentioned in my first
clarification:

MAKING TELESCOPES
http://www.eskimo.com/~billb/amateur/teles.txt

(My head still hurts from trying to cross my eyes with one of them
closed!)

I hope one of our researchers can find a positive answer for you,

revbrenda1st

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Clarification of Question by knowitall22-ga on 03 Jan 2003 21:09 PST

I have been asked to clarify my question but do not know who requested
clarification. Help?
knowitall22

Hi,

Not well versed in this field, but doesn't the fact that raptors have
50% binocular vision help to explain the telescopic aspect? If that's
the case, you have a system with two lenses when the signals meet for
processing.

bcguide-ga

bcguide: Hi! Raptors do have binocular vision, but so do we. I don't
understand your suggestion...do you mean only one eye would function
as a telescope and the optical images from both eyes would mix? I
doubt if you intended that. But how could both eyes acting
simultaneously perform two different functions? We weak-eyed humans
can focus on a near or a distant object, but not simultaneously.
Thanks,
knowitall22

http://www.stanfordalumni.org/birdsite/text/essays/Hawk-Eyed.html has
some interesting information on the subject. The bottom line though is
that a single lens can't act as a telescope - you must have a minimum
of 2, or 3 if you want the image to be the right way up.

Ian G.

iang: Hi, and thanks for the very interesting reference. In it, it is
said that hawks have a flattened lens and a rather long focal length,
producing the telescopic effect. Yet we brilliant humans need 2 or 3
lenses to make a decent telescope. I guess the substance of my
question is why can't we duplicate hawk vision with an optical
apparatus? Or has it been done?
Thanks,
knowitall22

revbrenda1st: An interesting comment. I had no idea the first galilean
telescopes had one lens. Very recently I read an account of Galileo's
development (not necessarily invention) of the telescope...it implied
two lenses. Perhaps it was in error. Be that as it may, in my long
life I have never seen a single lens telescope. My grasp of optics
could be written in several short sentences, but are you referring to
what is actually two lenses, convex and plano-convex, glued together
and regarded as one lens?
Thanks,
knowitall22

ABILITY OF VISION DEPENDS ON THE NUATURE OF THE MATERIAL WITH WHICH ,
THE EYE IS MADE OFF. First off all bird eyes do not function like
TELESCOPE. Birds do not find objects either bigger or closer. But they
can distinguish between two closely spaced objects,this is called
resolving power. the resolving power of birds is very high due to the
construction of the eye. So that they can see and undestand from long
distance.
sorry friend it is not a telescope, there is no magnification here.
byee

broadband: Hi! An extremely interesting comment...it makes sense. I
have seen references to the resolving power of avian vision, but also
numerous Internet articles describing it as telescopic. To the layman
(me) it seems intuitive that the amazing ability of raptors to spot
small prey at great distances implies telescopic vision, simply
because it is the only means with which humans could do the same. Your
alternate explanation clarifies the problem of the eye as a single
lens telescope.
Thanks,
knowitall22

Knowitall22 - Galilean telescopes had two lenses, not one. However, my
comment that you need 3 lenses for the image to be the right way up is
wrong - Galileo used a convex objective lens and a concave eyepiece.
The image is the right way up, though it doesn't make for a good
telescope. Modern opera glasses have the same arrangement.

Ian G.

There are several factors that give those birds high visual acuity. It
is best explained as a higher density of "sensors", similar in effect
to the difference in image quality between a 1Meg and a 5Meg pixel
digital camera.

The eye sees by projecting an image on the retina with a single lens.

http://www.earthlife.net/birds/vision.html
http://birds.ecoport.org/Identification/EBsight.htm
The number of receptors per square millimetre determines the degree of
visual acuity an animal has, the more receptors the higher its ability
to distinguish individual objects at a distance. In some birds such as
hawks, kingfishers and swallows, the eye has 2 fovea, one for sideways
viewing and one for forward viewing. In many raptors the fovea
centralis has far more rods and cones than in humans and it is this
which allows these birds their spectacular long distance vision. We
have about 200 000 receptors per mm2, sparrows however have about 400
000 while a Buzzard has an incredible 1 000 000 receptors per mm2. The
fovea itself can also be lens shaped increasing the effective number
of receptors per mm2 yet again. Buzzards for instance have distance
vision 6 to 8 times better than ours, part of this is a result of the
lens shaped central fovea which acts something like a times two
magnifying lens.

Bird Senses
http://www.biology.eku.edu/RITCHISO/birdbrain2.html
http://www.eeb.uconn.edu/courses/Ornithology/EEB281_09_Senses.htm
http://www.ornithology.com/lectures/Senses.html

Details on the Human Eye
http://www.mrothery.co.uk/vision/EyeNotes.htm

ulu-ga: Nice work! I accept your comment as the answer.
Thanks, 
knowitall22