Defective eyesight due to focusing(refractive)problems- ametropia:

It is often possible to correct ametropia completely with glasses

There are four general types of ametropia:

• Myopia (near sightedness) ,hyperopia or hypermetropia (far sightedness), astigmatism(asymmetrical focusing) and presbyopia (old sight) fig(2)  .

• The myopic individual usually has too long an eyebellor too much curvature of the cornea , distance objects come to focus in front of the retina, and the rays diverge to cause a blurred image at the retina .This condition is easily corrected with negative lens.

• A hyperopic eye has a near point further away than normal and use some of its accommodation to see distance objects clearly ,the usual cause of  hyperopic is too short eye ball , a positive lens is used to correct this condition fig(3).

• Inastigmatism ,the curvature of the cornea is uneven .Astigmatism cannot be corrected by a simple positive or negative lens, it is corrected with an  asymmetric lens in which the length is greater in one direction than in the perpendicular direction.