ASTIGMATISM - PRESBYOPIA
 
 
ASTIGMATISM
 
    If the cornea or the surface of eye is not perfectly spherical. In this situation the eye has different focal     points in different planes and an object is not focused clearly on the retina.
CORRECTION OF DEFECT
 
    ASTIGMATISM is corrected by using asymmetrical lenses which have different radii of curvature in     different planes
PRESBYOPIA
or
lack of accommodating
 
    At old age, the eye lens loses its natural elasticity and ability to change its shape and the ciliary muscles     weaken resulting in a lack of accommodation. This type of long sightedness is called "PRESBYOPIA".
CORRECTION OF DEFECT
 
This defect can be corrected by using convex lens for long sighted person and concave lens for short sighted person.
 
 
 
 
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