You must have seen people wearing specs. But why do they wear them? Let us find out. Some people complain that they cannot see the distant objects distinctly while some complain that they are unable to clearly see the nearby objects. They have blurred vision due to different refractive eye defects.

 

The 3 common refractive eye defects are:

 

1. Presbyopia, in which the eye loses the power of accommodation with aging. A person suffering from presbyopia finds it difficult to see the nearby objects and sometimes also the distant objects clearly.

 

2. Myopia or nearsightedness, in which a person cannot see the distant objects distinctly

 

3. Hypermetropia or farsightedness, in which a person cannot see the nearby objects clearly.

 

Let us learn about eye defects and their corrections in detail.

 

Presbyopia

 

Presbyopia is a condition in which the human eye loses its ability to focus, thus making it difficult to see nearby objects. Presbyopia is a natural process related to aging and is often referred to as �old sight�.

 

With aging, the gradual weakening of the ciliary muscles and the loss in flexibility and elasticity of the eye lens render it difficult to see the nearby object distinctly without using corrective eyeglasses.
Presbyopia is similar to hypermetropia, as in both cases the near point recedes to a distance more than 25 cm from the eye. 25 cm is the standard value assumed in all corrective optical instruments.

 

Causes of presbyopia

 

With aging, the increase in focal length of the eye lens leads to this condition. For correction of presbyopia, a person has to wear specs fitted with a convex lens of suitable focal length. Sometimes, a person may suffer from both myopia & hypermetropia.

 

In such cases, bifocal lenses are used as corrective lenses. The upper half consisting of concave lens is for distinct vision while the lower half consisting of convex lens facilitates clear near vision.

 

Myopia

 

Myopia or short-sightedness is the eye defect by virtue of which it can see the nearby objects clearly but cannot see the far-off objects distinctly. A myopic person can read or write comfortably but such a person faces difficulty in driving since his distant vision is not clear.

 

Generally, the far point for a normal eye is at infinity but for a myopic eye, the far point is nearer than infinity due to which the image of a distant object, instead of forming at the retina, forms in front of the retina. This happens because, in a myopic eye, the parallel rays from infinity are focused at a point in front of the retina thereby resulting in blurred vision.

 

This eye defect is attributed to mainly two causes. These are:

 

• Elongation of the eyeball, resulting in the adjustment of the eye such that the distance between the eye-lens and the retina is greater than that in a normal eye, and�.
• Excessive curvature of the eye lens, resulting in a decrease in the focal length of the eye lens.

 

Let us now discuss how a myopic eye can be corrected.

 

A myopic eye defect can be corrected by wearing specs fitted with a concave lens. A concave lens of suitable power produces divergence of the parallel rays passing through it so that the image of the distant object is formed on the retina.

 

Hypermetropia

 

Hypermetropia or long-sightedness is that the eye defect by virtue of which it can see the distant objects clearly but cannot see the nearby objects distinctly. So, a hypermetropic person can see any distant object clearly but finds reading and writing difficult.

 

This is due to the fact that the near point or the least distance of distinct vision which is 25 cm for a normal eye is much beyond that for a hypermetropic eye. Because of this, the image is formed much beyond the retina.

 

Let us now try to understand this better through this figure.

 

Here, in the normal eye, see the rays from the near point N focusing at the retina. On the other hand, in the hypermetropic eye, the rays from the normal near point N are focussed beyond the retina of the eye, thus shifting the near point from N to N�. This is the reason why the objects lying nearby appear blurred.

 

The possible causes responsible for this eye defect are:

 

• 1. Decrease in the length of the eyeball
• 2. The focal length of the eye lens is too long

 

How can a hypermetropic eye be corrected?

 

A hypermetropic eye defect can be corrected by wearing specs fitted with the convex lens of suitable power. A convex lens converges the rays of light to a near point farther from that of a normal eye, thus forming the image at the retina.

 

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