Defects of Vision: The gift of sight allows us to gaze at the wonders on this beautiful planet and will enable us to read, write, and work. However, our eyes need artificial help through glasses and lenses with time. This happens because of genetic factors, macular degeneration, or if our work requires us to look at screens for hours. Some people can see clearly when things are kept a reasonable distance away from their eyes, but others require glasses or lenses to see objects around them clearly. The improper convergence of images can explain these problems by the natural lens present in front of our eyes. Thus, using glasses to cover up for defects of vision is common around us, but let’s read in-depth about different types of these defects, why they occur and more.

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Defects of Vision: Meaning

The human eye is an optical instrument, just like a photographic camera. It captures a sight and forms a real image on the light-sensitive screen present at the back of the eye called the retina. A normal eye can see clearly within the range of \(25\,\rm{cm}\) (which is the least distance of distinct vision) to infinity. When the object is brought closer to the human eye, Then to maintain the image on the retina, the focal length of the human eye lens becomes shorter by the action of the ciliary muscles.

This property of the eye is called accommodation. However, If, for some reason, the eye lens can not focus clearly, the person will not be able to see the objects distinctly and comfortably. In addition, the vision may become blurred due to the refractive defects of the eye.

The four common defects of vision are:

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

Myopia

Also known as near-sightedness, Myopia is the defect of vision due to which a person can see nearby objects clearly but finds it difficult to see the far off objects distinctly.

It is caused by bulging cornea, due to which the radius of the curvature of the whole of the eye decreases. The focal length of the eye is decreased. The eye is unable to focus the image of distant objects at the retina, and thus, the image is formed near the eye lens.  Hence the name 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 leading causes of myopia are:

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, causing a hindrance to see 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: Myopia can be passed on from parent to their children and can develop due to diabetes. Often people with unattended cataract growths show signs of myopia too.

The far point of the eye having myopia is at a point \(F\) which is less than infinity. The eye can focus the rays coming from the person’s far point \(F\) on the retina. If the distant object can be made to appear as if it were at the far point \(F\) of this eye, then the eye can see it clearly. It is done by putting a concave lens that decreases the eye-lens converging power helping to form the image in front of the eye.

Hypermetropia

Hypermetropia is also known as far-sightedness. 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 \((25\,\rm{cm}\)). Such a person has to keep a reading material much beyond \(25\,\rm{cm}\) from the eye for comfortable reading. It 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. 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.

The leading causes of hypermetropia are:

1. Low converging or focusing power of crystalline eye-lens (because of its large focal length): The ciliary muscles attached to the eye-lens become weak and can not make the eye-lens thicker to increase its converging power.
2. Eye-ball being too short: A hypermetropic eyeball is too short, due to which the retina is at a smaller distance from the eye-lens. This condition also results in the formation of the image of a nearby object behind the retina.
3. Hypermetropia can be present in babies at the time of their birth. However, as they age, the eyeball lengthens to normal, and the defect is cured naturally.

The diverging rays of light coming from a nearby object \(O\) placed at the normal near point \(N\) are converged to form an image \(I\) behind the retina, due to which they cannot see the nearby object clearly. The image is formed behind the retina due to the low converging power of the eye-lens or eyeball being too short.

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 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 loss of elasticity of the eye lens is called Presbyopia. Sometimes, a person may have both farsightedness and short-sightedness. Such people often require bi-focal lenses. A common type of bi-focal lenses consists of 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.

The Gradual weakening of the ciliary muscles with old age is one of the main causes of Presbyopia. However, the decreasing flexibility or an increasing stiffness across the eye lens can also lead to Presbyopia. Both these causes occur with the eye’s natural ageing between \(40\) to \(60\) years of age. A person might have had a normal vision all his life, but with age, Presbyopia might develop.

Astigmatism

Astigmatism is a condition due to which a person can not see an object’s horizontal and vertical axis simultaneously with the same precision. It is a common vision problem caused by a fault in the shape of the cornea, resulting in an irregular curve. It can change the way light passes through the cornea and refracts onto the retina.  A refractive error causes it in the eye, making it unable to focus the image on the retina. As a result, people with this condition have blurry, fuzzy, or distorted vision. Astigmatism can result post an eye-related accident.

Astigmatism is generally caused due to the irregular shape of the cornea or due to a distortion in the eye lens. With developing technologies, it is possible to correct these refractive defects with contact lenses or surgical interventions.

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.

FAQs on Defects of Vision

Q.1. What is Myopia in the eye?
Ans: Myopia is also known as near-sightedness. It is the defect of vision due to which a person can see nearby objects clearly but finds it difficult to see the far off objects distinctly. Read this article to know more about Myopia.

Q.2. A student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected?
Ans: The student has Myopia. Myopia can be corrected by using glasses made from a concave lens of suitable focal length. 

Q.3. Why do we have two eyes for vision and not just one?
Ans: We need two eyes because a human being has a horizontal field of view of about 150°150° with one eye and about 180°180° with two eyes. Thus, two eyes provide us with a wider horizontal field of view. With one eye, the world looks flat, i.e., two-dimensional only. With two eyes, the view is three-dimensional, i.e., dimension of depth is added to our view.

Q.4. A person is advised to wear spectacles with convex lenses. What type of defect of vision is he suffering from?
Ans: He is suffering from Hypermetropia or far-sightedness.

Q.5. A person can comfortably read a book but finds it difficult to read the number on a bus parked 5m5m away from him. Name the type of defect of vision he/ she is suffering from? Which type of lens should he use in his spectacles to correct the vision?
Ans: The person can see nearby objects but finds it difficult to read the distant written words. The person has Myopia or short-sightedness. A concave lens should be used to correct the vision. 

Q.6. 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.