Definition of Astigmatism
Astigmatism: A common form of visual impairment in
which part of an image is blurred, due to an irregularity in the
curvature of the front surface of the eye, the cornea. The curve of
the cornea is shaped more like an American football or a rugby ball
rather than a normal spherical basketball.
Light rays entering the eye there are not uniformly focused on the
retina. Rays entering through the more-curved surface are focused
before the rays coming through the less-curved surface. The light is
focused clearly along one plane but is blurred along the other. The
result is blurred vision at all distances. Only part of what you are
looking at is in clear focus at any one time.
Astigmatism may be so
slight that it causes no problems. Almost everyone has some degree of
astigmatism. Significant astigmatism can cause headaches and eye
strain and seriously blur vision. Astigmatism may contribute to poor
school performance but is often not detected during routine eye
screening in schools. It is a refractive error, an error of
focusing, that may coexist with other refractive errors like near-
sightedness or far-sightedness.
Astigmatism is corrected with
slightly cylindrical lenses that have greater light-bending power in
one direction than the other. Using these lenses elongates objects in
one direction and shortens them in the other, much like looking into
a distorting wavy mirror at a circus The elongated figures in the
paintings of the great Spanish painter El Greco, it has been
suggested, might have been painted while he wore lenses to correct
astigmatism. This is clearly wrong since such lenses were not yet in
use in El Greco's day (1541-1614) and without them, what an
astigmatic saw would have been blurred, not elongated. X-rays also
show that El Greco first sketched more normal figures and then
elongated them for whatever effect, religious or artistic, he wished
to achieve.
Astigmatism was, in fact, not recognized until the 19th
century. Only thereafter were lenses devised to correct it. The word
"astigmatism" comes from the Greek "a-" (without) + "stigma" (point)
= "without a point" referring to there being no point of convergence
for the light rays on the retina.
Last Editorial Review: 9/21/1999
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