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 ?
Corneal astigmatism was measured by Bausch and Lomb manual keratometer preoperatively and at the 1st, 4th and 6-8th weeks post-operatively.
9D of corneal astigmatism where a limbal relaxing incision can be used at the time of surgery to reduce the astigmatism and increase the tolerance of the multifocal lens.
We therefore investigated the severity and prevalence of corneal astigmatism in leprosy patients undergoing pre-operative investigations before cataract surgery and present our results in this paper.
Table-1: Mean value of corneal astigmatism in both groups.
Independent sample't' test was also used to compare post-operative change in corneal astigmatism at 12th week between group A and group B.
Clinical pre-operative testing included slit-lamp exam and measurement of distant visual acuity, BCVA, spherical error, corneal astigmatism, spherical equivalent (SE), Kmax and CCT using Specular Microscope (Topcon SP 3000P, Topcon Corporation, Tokyo, Japan).
Participants with low corneal astigmatism were less likely to have a large amount of internal astigmatism.
3) It has been established that pterygium surgery decreases the pterygium induced corneal astigmatism at central 3 mm 10, but its effect on whole cornea remains unclear.
3--The mean posterior corneal power (PCP) and posterior corneal astigmatism (PCA): PCP (derived from the posterior axial curvature map) is the arithmetic mean of the pair of meridians 90 degrees apart with the greatest difference in average power, at a distance of 0.
If the incision is placed on the steepest meridian, the corneal astigmatism can be significantly reduced postoperatively4.
Objective: To determine mean change in visual acuity, corneal astigmatism and clinical appearance of pterygium after intralesional injection of 5-Fluorouracil.