resolution of retina

[T3D Peter Abrahams <telscope@xxxxxxxxxx>]

MIL-HDBK-141, page 4-9:
The arc subtense of a cone is about 1 arcminute, which is also the
resolution of the average eye when a point is the object of view, imaged at
the fovea.  A wire can be seen when it subtends one half arc second, or
120th the angular measure.  Horizontal or vertical lines are seen equally
well, but lines at 60 or 120 degrees to horizontal are about one third as
visible.  Two misaligned lines, like a vernier scale, are visible when
separated by 4 arc seconds.  Resolution drops off dramatically away from
the fovea, at 5 degrees away it is down to 25 percent,and at 10 degrees
from the fovea it is 7 percent.  All of these figures vary with contrast,
and are for objects that are not in motion.  A grating in motion can be
resolved if the details are more than about 2 arc minutes.
=================
These figures cannot be interpreted like tests done on a telescope.  The
image of a half arc second line on the retina is the diffraction pattern of
the line caused by the eye's pupil, visible to the brain if the background
is high contrast.  The resolution of that wire is not an optical function,
but a neural function.   A grating of wires that are that size, separated
by that distance or more, would not be resolvable. 
The brain, the optic nerve, and the retina, all perform information
processing that enhances input.  
In the retina, there are layers of nerve cell networks, some branches are
excitatory and some are inhibitory.  They provide feedback to the eye at a
faster rate than the brain/eye system.  This is used when tracking a moving
object, as an example.
=================
As noted in other posts, the photo receptors of the eye are behind the
nerves and fine blood vessels that serve them.   This does not seem to be
necessary, since the human fovea has no capillaries in front of it, and
cephalopods (octopus, squid) have their receptors at the front of their
retinas.  (Cephalopods also focus by moving the lens towards & away from
the retina.  They have rectangular pupils, and correct the astigmatism this
might produce by having two mutually correcting layers of cornea, and by
living in water, where the cornea has less refractive effect.)  
==============
The rods & cones of the retina are the rear most layer.  The nerves
carrying impulses from them exit to the front, and join the plexiform
layer, over the photo receptors.  The plexiform layer consists of these
nerves, small blood vessels, and connective tissue; all of which are very
fine & transparent.  I have not found a 'justification' for this design in
my reading.  Behind the photoreceptors is the choroid layer, which contains
larger blood vessels and acts to absorb light so it does not reflect back
to the photoreceptors.
This is relevant to binocular vision because the capillaries in front of
the retina are visible (using a pinhole, view the sky and move the pinhole
back & forth, the fine lines are the shadows of the capillaries.  By
carefull monocular viewing of the sky or a violet, translucent sheet, it is
possible to see the blood cells moving in the capillaries.  Raise your
heart beat with exercise and you can see the motion pulse.)  Although this
motion is invisible with normal observation, it is a source of 'noise' in
the visual field, along with floaters in the eye.  Binocular vision reduces
the effects of the 'noise' in each eye.

Correction of any errors in the above would be appreciated.
_______________________________________
Peter Abrahams   telscope@xxxxxxxxxx
the history of the telescope, the microscope,
    and the prism binocular


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