astigmatism


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astigmatism

(əstĭg`mətĭz'əm), type of faulty vision caused by a nonuniform curvature in the refractive surfaces—usually the cornea, less frequently the lens—of the eye. As a result, light rays do not all come to a single focal point on the retina. Instead, some focus on the retina while others focus in front of or behind it. The condition may be congenital, or it may result from disease or injury; it can occur in addition to nearsightednessnearsightedness
or myopia,
defect of vision in which far objects appear blurred but near objects are seen clearly. Because the eyeball is too long or the refractive power of the eye's lens is too strong, the image is focused in front of the retina rather than upon it.
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 or farsightednessfarsightedness
or hyperopia,
condition in which far objects can be seen easily but there is difficulty in near vision. It is caused by a defect of refraction in which the image is focused behind the retina of the eye rather than upon it, either because the eyeball is
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. The spherical lenses used to correct nearsightedness and farsightedness must be specially adapted to correct the out-of-focus plane of vision of the astigmatic eye. When the patient observes a pattern of straight lines placed at various angles, those running in one direction appear sharp while those in other directions (particularly at right angles to the sharp lines) appear blurred. A special cylindrical lens is placed in the out-of-focus axis to correct the condition. In many cases contact lenses are the most effective means of correcting astigmatism.
Astigmatism: astigmatism due to concave mirrorclick for a larger image
Astigmatism: astigmatism due to concave mirror

astigmatism

(ă-stig -nă-tiz-ăm) An aberration of a lens or mirror system that occurs when light falls obliquely on the system and is focused not as a single point image but as two perpendicular and separated lines. In the reflecting system shown in the illustration rays from points A and B on the mirror converge to the vertical line image ab; rays from C and D converge to the horizontal line image cd. The pencil of reflected rays, elliptical in cross section, cannot produce a sharp image anywhere along its path; the plane of optimum focus occurs between ab and cd where the pencil has its smallest cross section. Astigmatism is not as severe an aberration as coma.

astigmatism

[ə′stig·mə‚tiz·əm]
(electronics)
In an electron-beam tube, a focus defect in which electrons in different axial planes come to focus at different points.
(medicine)
A defect of vision due to irregular curvatures of the refractive surfaces of the eye so that focal points of light are distorted.
(optics)
The failure of an optical system, such as a lens or a mirror, to image a point as a single point; the system images the point on two line segments separated by an interval.

astigmatism

, astigmia
1. a defect of a lens resulting in the formation of distorted images; caused by the curvature of the lens being different in different planes
2. faulty vision resulting from defective curvature of the cornea or lens of the eye
References in periodicals archive ?
SynergEyes manufactures and markets hybrid contact lenses including the Duette family of contact lenses for astigmatism with Duette[sup.
The TECNIS Toric IOL decreases astigmatism while improving visual quality, with improved functional vision," said Kevin Waltz, O.
The correction of astigmatism with soft contact lenses.
Refractive Surgery--Procedures to correct myopia, hyperopia, astigmatism and presbyopia.
Recently, however, some companies are linking refractive surgery lasers to corneal topography machines, which results in surgery based entirely on the cornea's shape, thus disregarding the refractive astigmatism correction.
As long as correction of nearsightedness and some forms of astigmatism are the only approved uses for the laser, the keycards will allow you to load software only for those procedures.
Astigmatism is an eye condition in which the curve of the cornea is not entirely round in shape, making it difficult for the patient to clearly focus on an object and results in blurred vision.
This new lens is built on Alcon's industry leading AcrySof([R]) platform and is designed to provide cataract patients that have astigmatism a surgical option that delivers quality vision at all distances with increased spectacle independence.
K readings--these give the amount ot corneal astigmatism, allows prediction of lenticular astigmatism and the selection oft starting back optic zone radius (BOZR)
Medical Director and CEO of Evergreen Eye Centers located in Federal Way, Auburn, Burien and Enumclaw, has become the first eye surgeon in the Pacific Northwest to implant the Crystalens Trulign Toric Artificial Lens for correcting astigmatism and presbyopia.
As well as making reading hard, people with an astigmatism will have problems with both near as well as distant vision.
While hyperopia and astigmatism did not vary among race and ethnicity categories, myopia prevalence was higher at 38.