We use the following corrective lenses for normal eye defects. We consult an eye doctor or ophthalmologist to get prescription. The lens maker makes the spectacles or contact lenses.
Many people above 50 years get such defects. Some young persons also can get these defects, if there is some vitamin and mineral deficiency in their bodies. The muscles in the eye lens are not able to contract or expand suitably so that the focal length changes in order to form image on the retina. The retina is around 2.5 cm behind eye lens. Normally, depending on object distance (that we are focussing on), the curvature of eye lens is changed automatically by our eye.
1. Convex lenses :
People suffering from
farsightedness or hypermetropia cannot see nearby objects clearly -
rather, they cannot focus them clearly on retina. The image is formed behind the retina. For example, they may be able to see objects placed 1 meter to 20 meters or more. The least distance of distinct vision has increased.
these converging lenses are prescribed. Depending on requirement, plano
convex or double convex may be prescribed.
from shortsightedness or myopia focus objects from distance, at a
location in front of retina. So they are prescribed diverging
lenses. Depending on requirement, plano concave or double concave may be
The image is formed in front of the retina. For example, they may be able to see (distinctly focus) objects placed 25 cm to 10 meters only. The far point eye has decreased from infinity to 10 meters.
suffering from astigmatism have a trouble that they cannot focus correctly
the objects in all directions. Objects in vertical direction and
horizontal direction are not both focused correctly at the same time. Depending on requirement plano cylindrical or
total cylindrical lenses may be prescribed.