Is it possible to see with our eyes closed? We can hear, feel and smell the environment around us, but we must look at it with our eyes open to see it. Our eye is like a camera that is constantly recording. Our brain processes what we see as soon as we look at it. Sometimes there are Defects in Eye Vision due to many factors which will be discussed in this article.

The light rays coming from the object undergo refraction as they pass through our eye, and a real, inverted image is formed at the back of the eye. This inverted image is read by our mind, which straightens it, and hence we see the world as it is and not upside down.

Our eyes adjust to different environments, but sometimes, due to certain defects, our eyes may lose their power to adjust, also known as the power of accommodation. In that case, we have to supplement our sight with a pair of glasses or contact lenses that help us see properly. Continue reading to know more about this topic.

Defects of Vision

We often go to doctors to get our sight checked. The perfect vision for both our eyes should be \(\frac{6}{6},\) but sometimes a person faces issues in seeing the objects distinctly and comfortably due to loss in the power of accommodation of the eye. Sometimes, the vision becomes blurred due to the refractive defects of the eye. In all these cases, treatment is required to ensure a person can see correctly. There are four common defects of vision.

They are:
1. Myopia (Short-sightedness or Near-sightedness)
2. Hypermetropia (Long-sightedness or Far-sightedness)
3. Presbyopia
4. Astigmatism

Myopia

Nearsightedness, or myopia, is the defect of vision due to which a person can see nearby objects clearly, but find it difficult to see the far-off objects distinctly.

In a myopic person’s eye, there is a bulge around the cornea, causing a reduction in the radius of the curvature of the whole eye, and hence, the focal length of the eye is decreased. The eye becomes unable to focus the image of distant objects at the retina as the eyeball is longer (from front to back) than normal. This causes images to be focused in front of the retina instead of on the retina. Thus, the image is formed near the eye lens; that is why this eye defect is called nearsightedness or myopia. As a result of this defect of the vision, the distant objects look blurred. The maximum distance at which a myopic eye could see an object is known as its far point; beyond this far point, an image is formed near the eye lens instead of at the retina.

Myopia can be corrected with concave lenses. The lenses focus images farther back in the eye, so they fall on the retina instead of in front of it.

Causes of Myopia

It is caused due to:
1. High converging power of eye-lens (because of its short focal length): Due to the high converging of the eye-lens, the image is formed in front of the retina, and a person cannot see the distant objects.
2. Eye-ball being too long or cornea bulged: If the eyeball is too long, then the retina is at a larger distance from the eye-lens. In this case, the image is also formed in front of the retina even though the eye-lens have correct converging power.
3. Hereditary or due to uncontrolled diabetes or unattended cataract growths.

Correction

Myopic eyes do not diverge light rays from far off objects; hence, a focused image cannot be formed on the retina. Instead, the rays converge much before they reach the retina. Myopia or shortsightedness can be corrected by wearing spectacles containing a concave lens. When a concave lens of suitable power is used for the myopic eye, the concave lens first diverges the parallel rays of light from a distant object. Therefore, first, a virtual image is formed at the far point of the myopic eye. Then, since the rays of light appear to be coming from the eye’s far point, they are easily focussed by the eye-lens, and the image is formed on the retina. Thus, a concave lens is used for a myopic eye to decrease the converging power of the eye lens.

The focal length of the correcting lens \(=\) distance of the far point from the eye.

Hypermetropia

Hypermetropia is also known as farsightedness. A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly. The near point, for the person, is farther away from the normal near-point \(\left( {25\,{\rm{cm}}} \right).\) Such a person has to keep a reading material much beyond \(25\,{\rm{cm}}\) from the eye for comfortable reading. This is because the light rays from a closeby object are focussed at a point behind the retina. Thus, Hyperopia or Hypermetropia is when our eyes see distant objects clearly, but nearby objects appear blurry. It occurs when the eyeball is shorter than normal. This causes images to be focused in a spot that would fall behind the retina (if light could pass through the retina). Hyperopia can be corrected with convex lenses. The lenses focus images farther forward in the eye, so they fall on the retina instead of behind it.

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Causes of Hypermetropia

It is caused due to:
1. Low converging or focusing power of crystalline eye-lens (because of its large focal length)
2. Eye-ball being too short.
3. Hypermetropia can be present in babies at the time of their birth, but as they grow older, the eyeball lengthens to normal, and the defect is cured naturally.

Correction

The near-point of an eye having hypermetropia is more than \(25\,{\rm{cm}}{\rm{.}}\) Therefore, this defect can be corrected by putting a convex lens in front of the eye. When a convex lens of suitable power is placed in front of the hypermetropic eyes, the convex lens first converges the diverging rays of light coming from a nearby object near the eye, which is the virtual image of the nearby object is formed. Since the light rays now appear to be coming from the eye’s near point, the eye-lens can easily focus and form the image on the retina. Thus, a convex lens is used for hypermetropia to increase the converging power of the eye lens.

The hypermetropic eye has positive power. This indicates that the corrective lens required is convex. Such lenses are mainly used during reading or using laptops.

The focal length of correcting lens, \(f = \frac{{yd}}{{y – d}}\)
Where \(y = \) distance of the near point from the defective eye.
1. However, instead of using eyeglasses, contact lenses are preferable and safe to use and have the same effect.
2. Another way to correct this defect is to alter the refractive power by surgery like Lasik surgery.

Presbyopia

The eyes lose their power of accommodation with ageing. As people grow old, the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens results in hardening of the eye lens, making it more difficult for the eye to focus on close objects. This causes the near point to recede away in older people gradually. As a result, these people may find it difficult to see nearby objects distinctly without corrective eyeglasses. This defect of farsightedness caused by the loss of elasticity of the eye lens is called presbyopia. Sometimes, a person may have both farsightedness and shortsightedness. People suffering from presbyopia often require bi-focal lenses. Bi-focal lenses, in general, contain both concave and convex lenses. The upper portion consists of a concave lens to assist distant vision. The lower part is a convex lens to assist near vision.

Causes of Presbyopia

It is caused due to:
1. Gradual weakening of the ciliary muscles.
2. Decreasing flexibility or stiffness of the eye lens.

Both these occur with the eye’s natural ageing between \(40\) to \(60\) years of age, although they may have had normal vision throughout their lives.

Correction

Presbyopia defect is corrected by using bi-focal lenses, which consists of both concave and convex lenses. The upper portion consists of a concave lens. It facilitates distant vision. The lower part is a convex lens. It facilitates near vision. Such difference is not visible in progressive lenses, which also function similarly but does not have a line distinguishing the two lenses. Since the transition is smoother, it is more comfortable.

Presbyopia may also be corrected with eyeglasses or contact lenses having a convex lens if both myopia and hypermetropia are not present simultaneously.

Contact lenses used have each lens correcting one defect. People having both defects also use monovision. However, the practical perceptive may not be very good. Modified monovision contact lenses solve the problem of depth perception.

Astigmatism

Astigmatism is a condition caused by a refractive error in which the eye does not focus light evenly on the retina. This results in distorted or blurred vision at any distance. Astigmatism is a common vision problem caused by a fault in the shape of the cornea, resulting in an irregular curve. This can change the way light passes through the cornea and refracts onto the retina. As a result, people with this condition have blurry, fuzzy, or distorted vision.

With developing technologies, it is possible to correct these refractive defects with contact lenses or surgical interventions.

Causes of Astigmatism

1. Irregularly shaped cornea
2. Distorted lens

Types of Astigmatism

1. Corneal Astigmatism: This is due to the irregular shape of the cornea.
2. Lenticular Astigmatism: This is due to the distorted shape of the lens.

Correction

This defect can be corrected by using eyeglasses with cylindrical lenses oriented to compensate for the irregularities in the cornea. Usually, the cornea is spherically shaped, like a baseball. However, in astigmatism, the cornea is elliptically shaped, more like a football. Therefore, the lenses are shaped to counteract the shape of the sections of the cornea that cause the difficulty. Hence, only cylindrical lenses are used to correct astigmatism.

Other Common eye-related Problems

Colour – Blindness

The retina of our eye has a large number of light-sensitive cells having shapes of rods and cones. The rod-shaped cells respond to light intensity with different brightness and darkness as the cone-shaped cells respond to colour. In dim light, rods are sensitive, but cones are sensitive only bright. The cones are sensitive to red, green and blue colour of light to different extents.

Due to genetic disorders, some persons do not possess cone-shaped cells that only respond to certain specific colours. Such persons cannot distinguish between particular colour but can be seen well otherwise. Such persons are said to have colour-blindness. Driving licenses are generally not issued to persons having colour-blindness.

Cataract

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A yet another defect of the eye that usually comes in old age is the cataract. The medical condition in which the lens of a person’s eyesight becomes progressively cloudy results in blurred vision. It develops when the eye-lens of a person becomes clouded due to the formation of a membrane over it. It decreases the eye’s vision gradually and can lead to a total loss of eye vision. It can be restored after getting surgery. The opaque lens is removed, and an artificial lens is inserted in its place via operation. Any spectacle lenses cannot correct this defect.

Glaucoma

The eyes generate a clear fluid (aqueous humour), filling the space between the cornea and the iris. This causes fluid to filter out through a complex drainage system. This is the balance between the production and drainage of this liquid that determines the eyes intraocular pressure (IOP). Glaucoma is a disease that is caused by increased IOP, usually resulting from a malfunction in the eye’s drainage system. High IOP can also cause irreversible damage to the optic nerves and retinal fibres and, if left untreated, can result in a permanent loss of vision.

Age-Related Macular Degeneration (ARMD)

It is a degenerative condition of the macula (the central retina). The reason for its cause is the hardening of the arteries that nourish the retina. This deprives the retinal tissue of the nutrients and oxygen needed to function and causes a deterioration in central vision.

Summary

There are four common defects of vision. They are:

1. Myopia (Short-sightedness or Near-sightedness): is the defect of vision due to which a person can see nearby objects clearly, but find it difficult to see the far off objects distinctly. It is caused due to either high converging power of eye-lens (because of its short focal length or due to the Eye-ball being too long or cornea bulged. A concave lens is used for a myopic eye to decrease the converging power of the eye lens.

2. Hypermetropia (Long-sightedness or Far-sightedness): A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly. It is caused due to either Low converging or focusing power of crystalline eye-lens or it could be due to eye-ball being too short. A convex lens is used for hypermetropia to increase the converging power of the eye lens.

3. Presbyopia: This defect is caused by the loss of elasticity of the eye lens is called presbyopia. It is caused by either the gradual weakening of the ciliary muscles or due to decreasing flexibility of the eye lens. People suffering from presbyopia often require bi-focal lenses. Bi-focal lenses, in general, contain both concave and convex lenses. The upper portion consists of a concave lens to assist distant vision. The lower part is a convex lens to assist near vision.

4. Astigmatism: Astigmatism is a condition caused by a refractive error in which the eye does not focus light evenly on the retina. It is caused due to either irregularly shaped cornea or a distorted lens. This defect can be corrected by using eyeglasses with cylindrical lenses oriented to compensate for the irregularities in the cornea.

Frequently Asked Questions

We have provided some frequently asked questions here:

Q.1. What kind of corrective lenses are used to treat hypermetropia?
Ans: Eyeglasses with converging lenses provide the additional focusing power required for forming the image on the retina. Thus, we use the convex lens of suitable focal length to correct far-sightedness.

Q.2. Why cylindrical lenses are used to correct astigmatism?
Ans: Astigmatism is an image distortion that results from an improperly shaped cornea. Usually, the cornea is spherically shaped, like a baseball. However, in astigmatism, the cornea is elliptically shaped, more like a football. The lenses are shaped to counteract the shape of the sections of the cornea that are causing the difficulty. Hence, only cylindrical lenses are used to correct astigmatism. 

Q.3. What is Presbyopia?
Ans: The power of accommodation of the eye usually decreases with ageing. For most people, the near point gradually recedes away. As a result, they find it difficult to see nearby objects comfortably and distinctly without corrective eyeglasses. This defect is called presbyopia.

Q.4. What is myopia, and what are its causes?
Ans: The defect of an eye in which it cannot see distant objects clearly is called myopia. A person with myopia can see nearby objects. Myopia is caused due to:
1. High converging power of the lens
2. Eye-ball being too long 

Q.5. If a person cannot see an object clearly when it is placed at about \(25\,{\rm{cm}}\) away from him, he is suffering from:
a. Myopia
b. Presbyopia
c. Hypermetropia
d. Astigmatism
e. None of these
Ans: c. Hypermetropia
Hypermetropia is a defect in which a person is unable to see close objects. This happens because of the shortening of the eyeball or the loss of power of the eye lens.

We hope you find this article on Defects in Eye Vision helpful. In case of any queries, you can reach back to us in the comments section, and we will try to solve them.