For a college student with average vision the near point is not greater than 25 cm or about 10 inches. 1 Defects of vision and their correction. As a person s age advances the near point recedes as a result of declining power of accommodation. Eventually a person has difficulty with reading because the near point is too far from the eye. The lens cannot be made sufficiently converging to bring rays diverging from a point as near as 25 cm to a focus on the retina.
P is imaged on the retina.
N is imaged behind the retina and thus out of focus on the retina. However, the converging lens is only needed when viewing near objects, reading for example, and is not necessary when viewing distant objects. In fact, a person with far sight cannot see far object clearly when viewing them through the lens that allows then to view near object. A person having hypermetropia cannot see distant objects clearly since all rays entering the eye are focused behind the retina. The case for parallel rays entering the eye is illustrated in Fig. 4.