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 ?
More encouragingly, refitting lapsed astigmatic contact lens wearers with modern toric soft lenses achieves a higher success rate compared with a previous UK estimate from 2002 (94% versus 69%).
Astigmatic keratotomy involves one or two incisions on the surface of the cornea in a curved pattern.
Surgically Corrects Astigmatism: The LenSx[R] laser is FDA approved for astigmatic keratotomy, which doctors use to treat astigmatism.
All three instruments offer automatic focusing of the fundus image and provide correction of the spherical refractive error during testing, however, there is no option for correcting astigmatic refractive error.
SynergEyes, the leading high technology contact lens company for the independent eye care professional manufactures and markets the Duette family of contact lenses for astigmatic patients, including presbyopes, seeking exceptional vision.
The 23 chapters cover benefits, keratoplasty techniques, uses in cataract surgery, corneal crosslinking treatment, corneal inlays for presbyopia, the surgical treatment of keratoconus and LASIK ectasia, LASIK flap creation, advanced isovision, assisted refractive cataract surgery and presbyopia reversal, lamellar corneal resection, macular degeneration, analysis of the vitreous of diabetic patients, flap complications, astigmatic incisions, and other topics, including specific steps for techniques.
Zina was near-sighted and astigmatic, even though neither her parents nor her three siblings are similarly afflicted.
Typically, the music suggests extremes of climate or temperature--Zoviet France's "Daisy Gun" conjures up the polar twilight in Siberia, Total's "Six" is as astigmatic to the ears as staring into a blast furnace is for the eyes.
Their spiritual DNA matches the church's but they don't have an astigmatic view of morality and they aren't trapped in the heresy of incidentals.
This system compensates for astigmatic aberration and spherical aberration, and provides extremely high optical performance from infinity to 1:1 macro.
The astigmatic component of a patient's correction is usually corrected by accurate placement and adjustment of limbal incisions during surgery or by using a toric IOL where indicated.
He was short-sighted and astigmatic, which meant one eye pulled slightly to the side.