Defect of vision

A person with normal sight can see clearly the object lying between near point and far point. This distance is called range of vision. In normal condition, eye can adapt itself for objects at different distances but in practice a person may suffer from the following common eye defects.

1. Shortsightedness (Myopia): 

          A person suffering from this defect can see only near objects clearly but can't see the objects clearly lying at a far distance.

2. Farsightedness (Hypermetropia): 

       A person suffering from this defect can see only the distance objects clearly but cannot see nearly objects. 

3. Astimatism: 

          An eye suffering with this defect cannot focus on object lying at different planes (horizontal and vertical objects) at the same distance at the sometime so we cannot obtain any where a sharp image of the object.

4. Phorias: 

          It is a defect in the orientation of two eyes with respect to each other. It is due to the lack of balance among the muscles which hold the eye in their axes. It can be cured by putting prismatic lens in front of the eye.

5. Preshyopia: 

          This is the combination of myopia and hypermetropia. This type of defect arises in old age. The person suffering from this defect may need a converging lens for reading purpose and diverging lens for seeing at a distance. It can be corrected by using bifocal lenses.

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Farsightedness or Longsightedness or Hypermetropia or Hyperopia

A person suffering from hypermetropia can only see distant objects clearly but cannot see clearly an object lying closer than a certain distance. In other words, the distance of the near point 'N' of a hypermetropic eye from it is greater than least distance of distant vision. The rays of light from object at least distance of distinct vision are brought to focus behind retina at the point O as shown in fig alongside. This defect occurs when either:

i] the eye ball becomes shorter than the normal size, i.e distance between the eye lens and the retina becomes less.

ii] the focal length of the eye lens, when the eye is fully relaxed, become large.

Remedy:

Such a defect will be cured, if the eye can see clearly objects placed at near point 'N' i.e. at least distance of distinct vision. To correct for this defect, a convex lens of suitable focal length is placed close to the eye so that the rays of light from the object at least distance of distinct vision after refraction through the  lens appear to come from the near point 'N' of the hypermetropic eye as shown in fig below.

What causes long-sightedness?

The causes of hypermetropia are usually genetic (hereditary). Long-sightedness can occur at any age but it tends to become more noticeable above the age of 40 years. 

In rare cases, hypermetropia is caused by other conditions such as diabetes, small eye syndrome (microphthalmia), cancers around the eye and problems with the blood vessels in the retina. 

A particular type of age-related long-sightedness called presbyopia occurs because the lens of the eye becomes more stiff with age. (See separate leaflet called 'Presbyopia' for more information.)

What are the symptoms?

The main symptom is a difficulty with near vision. 'Tiring' of the eyes (asthenopia) is common and long-sighted people may have headaches and uncomfortable vision. 

There may be difficulties with seeing with both eyes (binocular vision) as the brain will tend to ignore signals coming from the most long-sighted eye. Amblyopia (lazy eye) or strabismus (squint) can also occur in hypermetropia. Long-sighted people may have difficulty with depth perception (3-dimensional vision).

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