astigmatism (redirected from mixed astigmatism)

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 nearsightedness or farsightedness. 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.

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astigmatism

Lack of symmetry in the curvature of the cornea or, rarely, the lens of the eye. The unequal curvatures spread light rays, preventing them from being sharply focused at a point on the retina, causing blurring of part of the image. The effect of astigmatism can also be produced by misalignment of the lens. Astigmatic vision is corrected by means of lenses (see contact lens, eyeglasses) that refract the light rays to the proper degree in the opposite direction of that produced by the defects in curvature.

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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

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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.