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 ?
As shown in Figure 1 the surgically induced astigmatism for most patients is within a 1 D of astigmatism; however, 3 patients had an astigmatic change larger than 1 D and in one patient the astigmatic change was larger than 2 D.
(1.) Mohammad Pakravan etal "Astigmatic outcomes of temporal versus nasal clear corneal Phacomulsification" Journal of Ophthalmic and Vision Research 2009 Vol.
The refractive power in astigmatic subjects showed no statistically significant difference between the MMC and non-MMC groups (P = 0.0613).
Surgically induced astigmatism (SIA) vector is the amount and axis of astigmatic change caused by surgery, which expressed as polar values (AKP, AKP+ 45) are the difference between the postoperative and preoperative polar values.
Also it is difficult to use in oblique astigmatic axis15 and further studies are needed to develop the normograms correlating the incision length, width and age of the patients with preexisting astigmatism to be corrected.
While a reduction in performance in uncorrected astigmats, if observed, could be due to difficulties introduced by astigmatic blur, we must rule out the possibility that some or all of any reduced performance in uncorrected astigmats is due to poor visual motor or perceptual development resulting from persistent blur during early childhood development or to astigmatism-related amblyopia.
Using the neutralization method on a lensometer, Phillips found that the spherical correction was nil (as expected) and that the astigmatic component matched my prescription exactly.
Nielsen, "Prospective evaluation of surgically induced astigmatism and astigmatic keratotomy effects of various self-sealing small incisions," Journal of Cataract & Refractive Surgery, vol.
With increased patient expectations and improved techniques incision part of surgery has undergone metamorphosis and surgeons have been playing a closer attention to astigmatic effects of cataract surgery.
If the telescope is astigmatic, the ellipse will change orientation by 90[degrees] when you pass through focus.
Several techniques exist to correct corneal astigmatism, including limbal relaxing incisions [17], opposite clear corneal incisions [18], excimer laser refractive procedures [19, 20], femtosecond laser-assisted astigmatic keratotomy [21, 22], and toric intraocular lens (IOL) implantation [2325].