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The eye produces vision in much the same manner as a camera
taking a picture.
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The Camera
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The
Eye
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Light
entering the eye is focused, first by the curved clear cornea
(the watch crystal that covers the front of the eye) and then by
the lens which is positioned just behind the iris. The
iris has muscles which open and close the central hole in the
iris, the pupil, to let in the correct amount of light
(different on a bright, sunny day than indoors or at night).
The
lens in the human eye is similar to the lens of a camera except
that it is not glass or plastic, but is actually a bag of clear,
gel protein. The lens is focused in the eye by a muscle,
which pulls on the strands, or ligaments that suspend the lens
behind the pupil to change its shape. Therefore the shape
of the lens can be changed to focus the eye for objects on the
horizon as well as objects right up under your nose!
The
retina lies flat against the inside back wall of the eye; it is
similar to the film in the camera in that it takes a picture or
"sees" the image focused on it by the lens in the
front of the eye. The retina in each eye sends the image
it generates through the optic nerve to the brain where the
images are combined into one image with the perception of depth.
In
summary, the picture is taken of a scene by each eye (both eyes
pointed at the same object!) and the brain interprets the image
in depth, with feedback control of the position, focus, and
pupil size for each eye.
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What
Causes Vision Problems?
When a person complains of vision problems, most often they
fall into one of three general categories:
Problems
with the pupil or focusing of one or both eyes that cause glare
or blur.
Problems with the detection of the image by the retina or
its transmission to the brain through the optic nerve that cause
smudges in the field of vision.
3.
Problems
with the brain in its attempt to combine the images from the two
eyes into one image with depth perception.
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Problems with Focus or Glare
Focus problems
cause the image to be blurred (fuzzy overall) and may result  from
the incorrect shape of either the cornea or lens (as in
near-sightedness or far-sightedness) or astigmatism (where the
elements may be slightly warped), all of which cause a poor
focus on the retina. Most of these conditions can be
improved with glasses or contact lenses that merely correct the
focus of the eye. If the cornea or lens is sufficiently
irregular that glasses or a contact lens cannot adequately
correct the focus (and the image remains blurred), then surgery
must be performed to correct the irregularity -- either corneal
surgery (sometimes requiring transplantation) or lens surgery
(removing the cataract or cloudy protein within the lens and
implanting a small plastic lens within the capsule to replace
the protein that is removed).
 Glare is created by the cornea or more often by the lens if it
becomes cloudy. As we all grow older (chronologically
gifted!), the clear gel protein in the lens becomes crystallized
and cloudy resulting in glare -- which we experience as a haze
that washes out the contrast of the image on bright sunny days,
or as flare seen around headlights at night. Glare can be
reduced with filtering lenses (yellow-amber lenses, orange
"blue blockers", gray lenses that reduce the light
level overall, or polarizing lenses that reduce the glare from
reflective surfaces). When glare is not reduced adequately
by such lenses, then surgery (most commonly cataract surgery) is
performed to replace the cloudy lens in the eye with a clear
plastic implanted lens.
Rarely,
glare and focus problems are caused by an iris that cannot
adequately close down the opening size of the pupil.
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Problems with Image Detection by the Retina and
Transmission of the Image though the Optic Nerve.
 Problems
of the retina and optic nerve most often cause loss of color or
loss of contrast. Most commonly the problem is not
observed over the whole field of vision of the eye but rather is
experienced in portions (causing smudges or "scotomas"
in the visual field). Our ability to see textures and hues
is based on the contrast sensitivity of the retina -- the
ability of the film in the camera to produce a good,
high-contrast picture with correct colors. It is also
dependent upon the ability of the optic nerve to transmit that
picture to the brain (and on the ability of the brain to
"see" the image).
 Blockages
of blood vessels, or a
"stroke" of the retina, optic nerve, or brain, can
cause a defect in a portion of the visual field blurring and
with loss of color and contrast. Inflammation or
degeneration of the retina can cause focal blur or loss of color
and contrast. Leakage of blood vessels in the retina (such
as may occur in diabetes) or scarring of the retina not only
causes smudge defects in the field of vision but also produces
distortions of the retina, causing distortions in the field of
vision (doors or windows have kinks or bumps).
 For
these conditions, focus is not the problem and glasses do not
improve the vision, since the problem lies with the retina and
its inability to detect and transmit the image. The
problem becomes much worse under conditions of reduced lighting
(i.e. the restaurant where the lighting is romantic but you
can't read the
menu!). Here the key to improving vision is to
increase the lighting on the object with focal lights that don't
reflect into the face and with filter lenses that reduce glare
and improve contrast. Magnification devices that magnify
the image and increase light also are of some help, provided the
visual field defect is not large or severe, or if it is, then
devices which assist the person in using the remainder
"peripheral" vision may be of help.
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Problems with Integration of the Images from
the two eyes into one Image with Depth
 If
the brain cannot integrate the two images (one
from each eye) into one image with depth perception, then
the person has double vision (often noticed as a ghosting of the
image or as two separate images) or problems with depth
perception (pouring liquids outside of the cup, hitting the
fender of the car against the garage door!). Double vision
is most often observed only when both eyes are open (and
disappears when one eye is covered) and occurs either because
the eyes are not lined up to look at the same object, or because
one of the images is distorted or of a different size than that
in the other eye. These problems often require neurologic
or retinal evaluation and sometimes require surgery to correct
(or if impossible to correct, then permanent patching of one eye
may be required to remove the distracting second image).
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