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lenses for nearsighted people are thin in the center but get thicker at the edge of that center zone. to correct farsightedness, lenses have a thick center that tapers down at the edge of the optical zone. because standard lenses are free to rotate as they sit on the eye''s surface, they can''t correct for astigmatism, a condition in which the cornea doesn''t have a spherical shape. astigmatic eyes require toric lenses, which have different lens powers along different axes of vision. recently, lens designers have produced contacts for astigmatism. by making the outer part of the lens, called the carrier portion, thicker on one side than the other, the manufacturer can ensure that the correction stays in line with the axis of irregular vision. as the wearer blinks and the lens moves, the thicker, heavier part stays at the bottom, ensuring that the lens is oriented correctly. these contacts have become widely available within the past 2 years.

lenses for nearsighted people are thin in the center but get thicker at the edge of that center zone. to correct farsightedness, lenses have a thick center that tapers down at the edge of the optical zone. because standard lenses are free to rotate as they sit on the eye''s surface, they can''t correct for astigmatism, a condition in which the cornea doesn''t have a spherical shape. astigmatic eyes require toric lenses, which have different lens powers along different axes of vision. recently, lens designers have produced contacts for astigmatism. by making the outer part of the lens, called the carrier portion, thicker on one side than the other, the manufacturer can ensure that the correction stays in line with the axis of irregular vision. as the wearer blinks and the lens moves, the thicker, heavier part stays at the bottom, ensuring that the lens is oriented correctly. these contacts have become widely available within the past 2 years.

another phenomenon associated primarily with hard lenses is "spectacle blur." when a hard lens sale is placed on the cornea, the surface topography of the eye is altered, so the patient may have visual disturbances with glasses immediately after removing the lens. spectacle blur, contact in general, is cleared in about 30 minutes, but use of more oxygen-permeable lenses minimizes the problem. advantages of hard lenses include favorable specific gravity (1.18-1.20) and refractive index (1.49-1.50), excellent visual acuity, durability, ability to withstand weak alkalis or weak acids, and no sensitivity reactions when placed on the cornea. hard lenses are also relatively resistant to the absorption of disinfectants, surfactant cleaners and environment pollutants; they are easy to handle during insertion and removal. disadvantages of hard lenses include a longer adaptation time, poor oxygen lens permeation and ocular discomfort.

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