Astigmatism

Picture of an image blurred by an astigmatism Astigmatism is a common type of visual problem that partly blurs an image. This is because there is irregularity in the curve of the front surface of the eye (the cornea). The cornea is curved more like a football (an American football, that is) or a rugby ball rather than the normal shape of a spherical basketball.

Light rays entering the astigmatic eye are not uniformly focused on the retina. Rays going through the more-curved surface are focused in front of 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 an individual with an astigmatism is looking at is in clear focus at any given time.

Astigmatism may be so slight that it causes no problems. Almost everyone has some degree of astigmatism.

Moderate astigmatism can cause headaches and eye strain. Severe astigmatism can seriously blur vision. Astigmatism can contribute to poor school performance but paradoxically it is usually not detected during routine eye screening in schools.

Astigmatism is a refractive error. It may be present along with other problems in refraction, such as near- sightedness or far-sightedness.

Astigmatism is corrected with slightly cylindrical lenses that have greater light-bending power in one direction than the other. These lenses elongate objects in one direction and shorten them in the other, much like looking into a wavy mirror at a circus.

The figures in the works of the great Spanish painter El Greco are remarkably elongated. It has therefore been proposed that El Greco may have worn lenses for astigmatism and the lenses led him inadvertently to elongate the forms in his pictures.

This proposal is clearly incorrect. Lenses were not yet in use for astigmatism 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 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." This referred to the fact that there is no point of convergence for the light rays on the retina.


Last Editorial Review: 10/28/2002




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