Myopia and hyperopia.
It seems that whenever people who wear glasses or contact lenses get together and discuss their vision, one of them says something like, “Nearby objects appear blurry to me, but I can’t remember if that means I’m nearsighted or farsighted.” Or, someone else may say, “My glasses allow me to see faraway objects more clearly, so does that mean I am farsighted?” Here we will explain the meaning of nearsightedness and farsightedness as we explore the basis of eye-focusing disorders.
The eye that focuses images correctly on the retina is said to have emmetropia, literally, “harmonious vision.”
Nearsightedness is formally called myopia (“short vision”). It occurs when the parallel light rays from distant objects fail to reach the retina and instead are focused in front of it. Therefore, distant objects appear blurry to myopic people. Nearby objects are in focus, however, because the lens “accommodates” (bulges) to focus the image properly on the retina. Myopia results from an eyeball that is too long, a lens that is too strong, or a cornea that is too curved. Correction requires concave corrective lenses that diverge the light rays before they enter the eye, so that they converge farther back. To answer the first question posed above, nearsighted people see near objects clearly and need corrective lenses to focus distant objects.
Farsightedness is formally called hyperopia (“far vision”). It occurs when the parrarel light rays from distant objects are focused behind the retina – at least in the resting eye in which the lens is flat and the cillary muscle is relaxed. Hyperopia usually results from an eyeball that is too short or a “lazy” lens. People with hyperopia can see distant objects clearly because their cillary muscle contract continuously to increase the light-bending power of the lens, which moves the focal point forward onto the retina. However, the diverging rays from nearby objects are focused so far behind the retina that even at full “bulge,” the lens cannot focus the image on the retina. Therefore, nearby objects appear blurry. Furthermore, hyperopic individuals are subject to eyestrain as their endlessly contracting cillary muscles tire from overwork. Correction of hyperopia requires convex corrective lenses that converge the light rays before they enter the eye. To answer the second question posed at the beginning of this essay, farsighted people can see faraway objects clearly and require corrective lenses to focus on nearby objects. Unequal curvatures in the different parts of the cornea or lens cause astigmatism. In this condition, blurry images occur because points of the light are focused not as points on the retina but as lines (astigma = not a point). Special cylindrically ground lenses or contacts are used to correct this problem. Eyes that are myopic or hyperopic and astigmatic require a more complex correction.
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