P3D hawk eyes

[Peter Abrahams <telscope@xxxxxxxxxx>]

'How Animals See' by Sandra Sinclair is a popular level but well-researched
book on animal vision. She says that predatory birds have a "flatter
cornea, which enables them to keep the total visual field in focus at the
same time" (compared to mammals, which have more spherical corneas.)  Some
birds have "two sets of ciliary muscles...to change focus: Brucke's muscles
change the shape of the lens.....some birds have a second set of ciliary
muscles, called Crampton's muscles,that can also change the shape of the
cornea.   The hawk has both types...".  The hawk buteo has 1,000,000
photoreceptors per square mm, compared to about 200,000 per square mm in
the human eye.  They also have a far higher ratio of nerve cells to
photoreceptors, closer to one nerve per photoreceptor than people.
If these Crampton's muscles encircle eye & squeeze it to become more of an
elliptical shape, that could be described in one place as 'changing the
shape of the cornea' and with some poetic license as a 'zoom lens'.  
_______________________________________
Peter Abrahams   telscope@xxxxxxxxxx
the history of the telescope, the microscope,
    and the prism binocular


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