Which defect of vision is represented in this case? Give reason for your answer.
The given figure represents hypermetropia because the image of an object placed at the near point is formed behind the retina.
u = -D, v = -y, f = ?
Required focal length, 
Presbyopia: A person suffering from presbyopia can see the distant objects distinctly but, cannot see the near-by objects clearly. In this case, power of accomodation usually recedes with ageing.
Stiffening of the ciliary muscles results in losing the flexibility of eyelens. Hence, accomodating power of the lens decreases.
People who suffer from both myopia and hypermetropia require bi-focal lenses. The upper part of the bi-focal lens is a concave lens used for distant vision, while its lower part is a convex lens used for reading purposes.
These defects can also be corrected via surgeries or by using contact lenses.
Astigmatism: It is a defect of vision in which a person cannot simultaneously see both the horizontal and vertical views of an object with the same clarity.
Cause of astigmatism: This defect occurs when the cornea of the eye is not perfectly spherical. This results in objects in one direction being well focussed while those in perpendicular direction are not well focussed.
Correction of astigmatism: Astigmatism can be corrected by using cylindrical lenses. They have different curvatures in horizontal and vertical directions and so they can be oriented suitably to compensate for the irregularities in the cornea, as shown in Fig.
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