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Hyperopia
Farsighted means you can�t
see things near. Why? The cornea does not have enough
curve or you have a short eyeball. Tough luck. In this
situation the focal point rests behind the retina. When
light enters the retina, blurring again takes place
because the images should have been focused before
getting to the retina, not after.
Astigmatig
If you suffer astigmatism
chances are your eyeballs have a football-like shape
when they should be more spherical. Instead of focusing
on one point, light entering the eye focuses on several
and you get blurring images.
You can be both astigmatic
and near-sighted or far-sighted, the latter requiring bi-focals.
Most people address these problems with corrective
lenses�glasses or contacts�but laser surgery can allow
you the opportunity to see without glasses if you are
nearsighted or astigmatic. One laser device has been
approved for use to correct farsightedness, but less
research exists on the results in farsighted people.
Presbyopia
Among the 40-and-older
crowd, it�s a rare individual who doesn�t need some
visual enhancements, even if they�re just off-the-rack
reading glasses from the drug store. Farsighted folks (hyperopes)
suddenly find that they need longer arms to read the
newspaper or a restaurant menu. Nearsighted folks (myopes)
may be removing their glasses more frequently to read
things up close.
The increasing difficulty
most of us experience as we age is due to a condition
known as presbyopia. The effects of presbyopia make us
increasingly farsighted as we get older.
Fields of Vision
Seeing an object and
recognizing it for what it is involves image processing
by cells in the retina and brain. Three type of cells
are involved in this processing: simple, complex and
hypercomplex. Simple cells respond best to a clearly
defined slit of light (or dark) or to edges between dark
and light areas, but the images have to be exact
locations in the receptive field of the cell and in the
proper orientation. Complex cells respond to slits and
edges as well, but they generally react to any location
or orientation of the image. The hypercomplex cells
respond best to even more specific features such as
corners, angles and lines of exact length, orientation
and location. The area which encompasses the sight of
both eyes is called the "field of vision". Capturing an
object, whether standing still or moving, and converting
it to nerve signals is only part of the process of
seeing. In order for us to see, the brain must analyze
the signals and draw meaningful information from them.
Some nerve signals are passed from the retina to the
temporal lobes of the brain, near the sides of the head,
and these contribute to the processes of visual
recognition and memory. Another section of the brain
acts to control visual attention. Objects on the
periphery of vision are detected and the head and eyes
are instructed to turn and bring the object into focus
inside the visual field. Another set of nerves carries
signals to the cerebellum, the part of the brain
responsible for muscular coordination. For full depth
perception, the stereoscopic vision by two eyes is a
necessity. Each eye views a scene from a slightly
different angle, and the fusion of these two images
gives a three-dimensional effect. With objects less than
200 feet away, vergence comes into play, so that each
eye is angled in a little to focus directly on the
object; the larger the object, the closer it is. When
objects are moving, the brain knows that close objects
pass faster than those farther away, and it can,
therefore, compare the apparent speeds. Perfect 20/20
vision will not be enough to pass an eye test given to
military pilots. It also involves "contrast sensitivity."
One must be able, for example, to see a white cat
walking in the snow. |
Myopia