Akademik

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.
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1. A lens or optical system having different refractivity in different meridians. 2. A condition of unequal curvatures along the different meridians in one or more of the refractive surfaces (cornea, anterior or posterior surface of the lens) of the eye, in consequence of which the rays from a luminous point are not focused at a single point on the retina. SYN: astigmia. [G. a- priv. + stigma ( stigmat-), a point]
- a. against the rule a. when the greater curvature or refractive power is in the horizontal meridian.
- compound hyperopic a. a. in which all meridians are hyperopic but to different degrees.
- compound myopic a. a. in which all meridians are myopic but to different degrees.
- corneal a. a. due to a defect in the curvature of the corneal surface.
- hyperopic a. that form of a. in which one meridian is hyperopic and the one at a right angle to it is without a refractive error. SYN: simple hyperopic a..
- irregular a. a. in which different parts of the same meridian have different degrees of curvature.
- lenticular a. a. due to defect in the curvature, position, or index of refraction of the lens.
- mixed a. a. in which one meridian is hyperopic while the one at right angle to it is myopic.
- myopic a. that form of a. in which one meridian is myopic and the one at right angle to it is without refractive error. SYN: simple myopic a..
- a. of oblique pencils an aberration occurring when a bundle of light rays strikes a refracting medium in some other direction than parallel to the axis of the lens.
- regular a. a. in which the curvature in each meridian is equal throughout its course, and the meridians of greatest and least curvature are at right angles to each other.
- simple hyperopic a. SYN: hyperopic a..
- simple myopic a. SYN: myopic a..
- a. with the rule a. when the greater curvature or refractive power is in the vertical meridian.

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astig·ma·tism ə-'stig-mə-.tiz-əm n
1) a defect of an optical system (as a lens) causing rays from a point to fail to meet in a focal point resulting in a blurred and imperfect image
2) a defect of vision due to astigmatism of the refractive system of the eye and esp. to corneal irregularity compare EMMETROPIA, MYOPIA

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n.
a defect of vision in which the image of an object is distorted, usually in either the vertical or the horizontal axis, because not all the light rays come to a focus on the retina. Some parts of the object may be in focus but light from other parts may be focused in front of or behind the retina. This is usually due to abnormal curvature of the cornea and/or lens (see refraction), whose surface resembles part of the surface of an egg (rather than a sphere). The defect can be corrected by wearing cylindrical lenses, which produce exactly the opposite degree of distortion and thus cancel out the distortion caused by the eye itself.
• astigmatic adj.

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astig·ma·tism (ə-stigґmə-tiz-əm) [a-1 + Gr. stigma point] an error of refraction caused by unequal curvature of the refractive surfaces of the eye, so that a point source of light cannot be brought to a point focus on the retina but is spread over a more or less diffuse area. This results from the radius of curvature in one plane being longer or shorter than the radius at right angles to it. astigmatic, astigmic adj

Astigmatism: the appearance of lines as seen by (A) the normal eye and (B) the astigmatic eye.


Medical dictionary. 2011.