The normal defects of vision are short sightedness and long sightedness. short sightedness means that the  person haing this can see near things clearly,and cant see far away thingsclearly.its also named as myopia or nearsightedness.its usually happens in  smllchildren. this can becorrected by using concave lens of suitable image is formed in front of the rectina of the eye. oldpeople who are above 45 yrs of age usually suffer from long sghtedness.this that thet person having this can see far away things clearly and cant see near things clearly. to correct this they have  to use specs with convex lens of suitable power.this is also named as farsightedness or hypermetropia. Through laser surgery we can avoid the usage of specs for both the defects.another defect is astigmatism. this can be corrected by operation
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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.

So these converging lenses are prescribed. Depending on requirement, plano convex or double convex may be prescribed.

2. Concave lenses:
         People suffering 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 prescribed.

  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.

3. Cylindrical lenses:
          People 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.

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