astigmatism

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Related to astigmatic: astigmatic keratotomy

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 ?
2) repeated keratometry measurements (preferably computerized video-keratometry) at different intervals, to ascertain the astigmatic changes over time and (3) use of different sizes of CCI incisions (3.
Stanko made a name for himself on the 1966 recording Astigmatic, an album of tough, asymmetrical themes, alternately abstract and lyrical, that still stands as one of the most important European jazz records ever made.
The Food and Drug Administration has approved it for use in refractive eye surgery in nearsighted and astigmatic patients and will soon approve it for farsighted patients.
A new ray path test for aspheric optics developed just to analyze these mirrors, has discovered regions that are optically well formed to a fraction of the wavelength of visible light alongside of highly astigmatic regions.
It wasn't until relatively recently I came to believe that my church's estimation of me was also flawed, astigmatic.
Newer and smaller implants, astigmatic and bifocal implants constantly improve treatment options," says Dr.
Such sloppy thinking lies behind Cardinal Mahony's astigmatic vision of the Church.
Letter writers, the fat-fingered or the astigmatic.
At birth, babies are hyperopic and often astigmatic.
Astigmatic eyes require toric lenses, which have different lens powers along different axes of vision.
In A Distant Episode, a collection of 24 perfectly written stories, Americans abroad (and the occasional Canadian) can't hear or see their surroundings--sometimes literally, as with the astigmatic Pastor Dowe--and continually get themselves into deep trouble that they can't begin to understand.
He referred, for example, to the astigmatic dusk, an archipelago of small rugs, an octave of good teeth.