Defects of vision can make it hard to see things clearly. When the retina in your eye doesn't focus correctly on an object, it can cause blurriness or distortion. There are many things that can cause vision defects. Figure 1 shows a diagram of a healthy eye where the focal point lands perfectly on the retina. The focal point is where light rays or waves meet after being reflected or refracted at the iris.
Myopia is a common vision defect that makes it hard for people to see distant objects clearly. If you have myopia, you can still see close objects without any problems. This defect happens when the focal point falls in front of the retina instead of on it. There are two main reasons why this happens.
The first reason is due to the high converging power of the eye's lens. This causes blurry images of distant objects because the ciliary muscles are not relaxed enough, causing the lens to thin out and decrease the converging power. The second reason is due to an irregularly long shape of the eyeball, causing the focal point to be in front of the retina instead of on it (see figure 2). This makes it hard for the eye to see distant objects clearly because there is a higher distance between the retina and the eye lens, making the image blurry and out of focus.
To correct myopia, the distance of the distant object needs to be adjusted to appear closer to the far point of the eye, which would allow the eye to see the object clearly. This is done by using a concave lens in front of the eye in the form of glasses or contact lenses. This divergence of the light rays coming from the distant object helps to increase the focal length, which is the distance between the centre of the curved lens and the focal point. The concave lens decreases the converging power of the eye lens, forming the image of the object on the retina as shown in figure 3. If the myopic eye is untreated, the image would form in front of the retina instead of on it.
The power of the lens needed to correct myopia is calculated mathematically using the formula below:
Power needed = (v - u) / f
Here, u is the distance of the distant object in metres, which is taken as infinity for the myopic eye, v is the far point in metres (the maximum clear vision distance), and f is the required focal length. The far point of an eye is 90 cm away from the eye. The power of the lens is set so that its focal length is equal to the far point of the eye. The required power is measured in dioptres (D), which is a measure of the degree of myopia. The higher the degree, the greater the elongation of the eye and the blurriness of images.
Hypermetropia is a common vision defect that it hard to see nearby objects clearly while still being able to see distant objects without any problems. Usually, objects that are more than 25 cm away from the eye can be seen clearly There are two main reasons why hypermetropia occurs.
The first reason is due to low converging power. In this case, the ciliary muscles are weak and unable to thicken the eye lens when needed to increase its converging power. This causes the image of close objects to form behind the retina instead of on it. The other reason is due to the shape of the eyeball. If the eyeball is too short, the retina is closer to the lens, causing the image of nearby objects to form behind the retina (see figure 4), resulting in blurry vision.
To correct hypermetropia, a convex lens is used to increase the converging power of the lens and bring the point forward to the retina. This helps to form a clear image nearby objects on the retina. The power of the lens needed to correct hypermetropia is calculated using the same formula asopia:
Power needed = (u - v) / f
Here, u is the distance of the near object in metres, which is taken as 25 cm for hypermetropic eyes, v is the near point in metres (the closest point at which a clear image can be seen), and f is the required focal length. The required power is measured in dioptres (D), which is a measure of the degree of hypermetropia. The higher the degree, the greater the weakness of the eye lens and the blurriness of nearby objects.
Convex lenses are used to correct hypermetropia, also known as long-sightedness. The light rays from near objects are bent slightly inward by the convex lens, which gives the eye additional focusing power. This helps to bring the image of the object closer to the near point N of the eye. Depending on the power of the lens used, the converging power will be increased accordingly.
When convex lenses are used, the image is formed on the retina instead of behind the retina. The normal near point of the eye is 25 cm, while the near point of a hypermetropic eye is more than 25 cm.
Astigmatism is a type of vision defect that occurs when the retina or the cornea of the eye is uneven or asymmetric, causing some images to be sharper than others. The irregular shape of the cornea or the lens causes the rays of light that enter the eye to focus at different points, resulting in blurry or distorted vision.
Astigmatism can be corrected using cylindrical lenses, which have different powers in different meridians. The power of the cylindrical lens helps to compensate for the uneven curvature of the cornea or the lens, allowing the light to focus on the retina at the same point.
The power of the cylindrical lens is calculated using the same formula as for spherical lenses, but with an additional term for the cylindrical power. The cylindrical power is represented by the letter C and is measured in dioptres. The cylindrical power corrects the difference in the curvature of the cornea or the lens between different meridians.
Astigmatism can also be corrected using toric lenses, which have different powers in different parts of the lens. Toric lenses are designed to fit the shape of the eye and correct the uneven curvature of the cornea or the lens.
In some cases, refractive surgery may be recommended to correct astigmatism. This can include procedures such as LASIK or PRK, which reshape the cornea to correct the uneven curvature and improve vision.
Cataracts are a common vision condition that can occur as a result of aging, injury, or other factors. They occur when the natural lens of the eye becomes clouded, causing vision to become blurry or cloudy. This can make it difficult to see clearly and can even lead to vision loss if left untreated.
The most common treatment for cataracts is surgery. During cataract surgery, the cloudy lens is removed and replaced with an artificial lens. This can restore clear vision and improve overall quality of life. Cataract surgery is a safe and effective procedure, with a high success rate and minimal side effects.
In some cases, cataracts may also be treated using specialized eyeglasses or contact lenses. These lenses can help to correct the effects of the cataract and improve vision. However, they are not a permanent solution and will not eliminate the cataract itself.
Regular eye exams are important for detecting cataracts early and beginning treatment as soon as possible. If you are experiencing blurry or cloudy vision or other symptoms of cataracts, it is important to speak with an eye doctor right away to determine the best course of treatment for your individual needs.
Defects of Vision and Their Correction: Presbyopia
Presbyopia is a common age-related vision condition that affects people over the age of 40. It occurs when the ciliary muscles in the eye weaken and lose their ability to focus properly on nearby objects. This can result in difficulty reading, working on a computer, or performing other activities that require close vision.
Presbyopia often occurs in conjunction with other vision conditions such as myopia, astigmatism, or hypermetropia. In these cases, corrective lenses such as bifocals, trifocals, or progressive lenses may be prescribed to address both conditions simultaneously.
There are also several surgical options available for treating presbyopia, including the use of intraocular lenses or corneal inlays. These procedures can help to restore near vision and reduce the need for corrective lenses.
Regular eye exams are important for detecting presbyopia and other vision conditions early on. By addressing these issues promptly, individuals can improve their quality of life and maintain good eye health as they age.
Presbyopia can sometimes appear in conjunction with myopia or hypermetropia, which would then require bifocal lenses that use concave or convex lenses at the top and the bottom of the spectacles, respectively. As presbyopia is associated with age, the formula for correction is given with respect to age or depending on the distance requirements, as seen below where f is the focal length in mm, and D the dioptre of the lens:
Expected amplitude (D) = 18.5 - 0.3 ⋅ (age in years)Maximum amplitude (D) = 25 - 0.4 ⋅ (age in years)Minimum amplitude (D) = 15 - 0.25 ⋅ (age in years)
Defects of Vision and Their Correction - Key takeaways Defects of vision are conditions that prohibit the eye from normal vision. The most common eye defects include myopia, hypermetropia, astigmatism, presbyopia, and cataract.These defects can be corrected by utilising lenses to correct the distance between the focal length and the retina or to correct cornea irregularities with laser surgery or lenses. Convex, concave, and bifocals are lenses that treat hypermetropia, myopia, and presbyopia, respectively.
What are the common defects of vision?
Myopia, astigmatism, presbyopia, hypermetropia, and cataract.
What is the corrective vision for the short-sighted defect?
Concave lenses are used to correct short-sighted vision.
What are the two main defects of vision, and how can they be corrected?
Myopia and hypermetropia are the main defects of vision. They are corrected by using spectacles or contact lenses with concave or convex lenses.
What is the best eye correction procedure?
LASIK surgery, which is a common term for all types of laser eye surgery, is considered a very successful procedure to correct various types of vision defects.
What is normal vision?
Our vision is considered to be normal when objects can be seen from 20 feet of distance.
