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.