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 ?
Other Studies have included more parameters like mean K, and axis of astigmatism as well15,16 but we in our study have limited ourselves to steep K and dioptric power of astigmatism only.
Vector Analysis of Astigmatism. Vector analysis according to Naeser [26] was used to calculate the surgically induced corneal astigmatism (SICA), the difference in TCA between preoperative and postoperative measurements, and the error in refractive astigmatism (ERA), defined as the difference between the observed and the targeted postoperative refractive astigmatism.
Various studies have shown that the general astigmatism in a normal population ranges from 0.89 to 1.25 D.
Cases with astigmatism were classified according to their SE (Table-I and II) and the commonest type was found to be the compound myopic astigmatism.
An exhaustive attempt was made to precisely quantify the pre-existing astigmatism by using manual keratometry (Bausch & Lomb model), auto refractokeratometry, understanding the previous prescriptions, comparing with the current retinoscopy etc.
The corneal (keratometry, corneal volume, anterior and posterior astigmatism, corneal apex, and thinnest-point corneal pachymetry) and anterior chamber parameters (anterior chamber depth, anterior chamber volume, and chamber angle) as evaluated by Pentacam are summarized in Tables 2 and 3, respectively.
Apparently my astigmatism was at the 'front and back'-meaning, aside from the abnormal curve of my cornea, I had an irregularly shaped lens, which is located at the back of the cornea.
Eye 7 hospitals almost exclusively use the EVO Visian ICL, manufactured by STAAR Surgical, Switzerland, which is customized to the unique refractive error of the patient and can also correct astigmatism.
"When the cornea has an irregular shape, it is called corneal astigmatism. When the shape of the lens is distorted, you have lenticular astigmatism.
Many studies have demonstrated that some ocular biometric characteristics, such as increased AL, reduced corneal curvature, and high corneal astigmatism, have potential diagnostic values in MFS patients [8-10, 19-22].
In 2016, toric lenses made up 32% of new lens fits, (1) while other research reveals that up to 47% of the spectacle wearing population has 0.75D or more astigmatism in at least one eye; (2) this suggests that some patients are not wearing their optimal refractive correction in contact lenses, although others will have been fitted with an aspheric design to mitigate for low astigmatism.