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Defects of Vision and their Correction

A person with normal eyes can, by virtue of accommodation, see clearly all objects that are at a distance greater than about 25 cm from the eye. In other words, the far point of a normal eye is at infinity and its near point is at about 25 cm from it. If due to certain abnormalities the is unable to accommodate itself to various distances, then the eye is said to be defective. A defective eye's far point may be closer to the eye (than infinity) or its near point may be farther away from the eye (farther than 25 cm). There are two common defects of the eye called myopia and hypermetropia.

1.Myopia (or Shortsightedness or Nearsightedness)

A person suffering from myopia (or shortsightedness) cannot distinctly see distant objects but can easily see near objects. The image of a distant object is not formed on the retina but in front of it. Consequently distant objects are not distinctly visible. In other words, his/her far point is not at infinity. He/She cannot clearly see objects lying beyond a particular point called his/her far point P. Object lying between his/her far point P and his/her near point N are made distinct by the process of accommodation.

Myopia is due to any one or both of the following factors:

(i) The lens may be thicker (more converging) than the normal eye lens.

(ii) The eye ball may be elongated. Due to elongation, distance between lens and retina becomes more than that for normal eye.

The concave lens kept just in front of the eye, receives distant parallel rays and diverges them. On eye lens, the rays fall as if coming from far point of the defective eye. The eye lens focuses them at retina. In this way, the concave lens is used to make a virtual image of distant (out of range) object within range of vision.


Hypermetropia (or Farsightedness or Long-sightedness)

A person suffering from hypermetropia (or farsightedness) can see distant objects well but nearby objects are blurred. His/Her eye lens is too thin or his/her eyeball is too short. The near point N of a normal eye is about 25 cm from the eye. The near point P of a hypermetropic person is farther away, so he/she cannot see objects that lie between near point N and far point P. The image of the object placed at near point N is formed behind the retina of a farsighted person.

This defect can be corrected if the rays from an object placed at N appear to come from near point P of the farsighted person. It is clear that a convex (converging) lens of suitable focal length will serve this purpose. The rays from the object at N, after refraction through the convex lens, are refracted. These rays appear to diverge from the point P and hence, after refraction through the eye lens they converge to a point on the retina. Thus, the object at N is made distinctly visible by using a convex spectacle lens.


Presbyopia (Greek word "presbyteros" meaning "elder") is the eye's diminished ability to focus that occurs with aging. It arises from the loss of elasticity of the crystalline lens, although changes in the lens's curvature from continual growth and loss of power of the Ciliary muscles (the muscles that bend and straighten the lens) have also been postulated as its cause.
Presbyopia is not a disease as such, but a condition that affects everyone at a certain age. When people develop presbyopia, they find they need to hold books, magazines, newspapers, menus and other reading materials at arm's length in order to focus properly. When they perform work which requires the close scrutiny of the eye, such as embroidery or handwriting, they may have headaches or eyestrain, or feel fatigued.
Eye glasses with bifocal or Progressive Addition Lenses (PALs) are the most common correction for presbyopia. Bifocal means two points of focus: the main part of the spectacle lens contains a prescription for nearsightedness or farsightedness, while the lower portion of the lens holds the stronger near prescription for close work.
Progressive addition lenses are similar to bifocal lenses, but they offer a more gradual visual transition between the two prescriptions, with no visible lines between them. Reading glasses are another choice. Unlike bifocals and PALs, which most people wear all day, reading glasses are typically worn just during close work.

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