Vision or eyesight is the ability to see. This is possible when the retina of the eye focuses correctly on an object. When this is not possible, objects may seem blurred or distorted, which is known as a defect of vision. There are several causes of defects in vision. As shown in the diagram in figure 1, in an eye with no defects, the focal point falls on the retina.
The focal point can be defined as the point where light rays or waves meet after they have been reflected or refracted at the iris.
Fig. 1 - A diagram of vision without any defects
Defects of vision and their correction: myopia or short-sightedness
Myopia is a defect of vision that affects a person's ability to see distant objects clearly. Those with myopia can see close objects clearly.
This defect occurs when the focal point falls in front of the retina instead of on it. This can happen for two reasons:
- The first is the high converging power of the eye’s lens. In this case, the ciliary muscles are not relaxed enough, which causes the lens to thin out and for the converging power to decrease. This causes blurry images of distant objects.
- The other factor is 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 results in a higher distance between the retina and the eye lens, which means that the eye is unable to see distant objects clearly. They appear blurry because the image is out of focus due to the long distance between the retina and the focal point.
Fig. 2 - Diagram of myopia vision
Myopia or short-sightedness correction
Correcting myopia involves adjusting the distance of the distant object so it appears closer to the far point of the eye, which would allow the eye to see the object clearly. This can be achieved by utilising a concave lens in front of the eye in the form of glasses or contact lenses.
This involves divergence of the light rays coming from the distant object, so they appear to come from the eye’s far point, which helps to increase the focal length. This 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. It illustrates that the image is created on the retina when the concave lenses are used compared to in front of the retina if the myopic eye is untreated.
Fig. 3 - Myopia correction vs myopic eye
The power of the lens is set so that its focal length is equal to the far point of the eye. The required focal length or power needed to correct myopia is calculated mathematically using the formula below.
If the focal length f is known, then the power p can also be estimated as they are inversely proportional, as shown below. The 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.
\[\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\]
\[P[D] = \frac{1}{f}\]
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. What power of lens is needed to correct its vision?
In myopia, the distance to the far object u is infinity, and in this case, v = -90 cm.
\[\frac{1}{f} = \frac{1}{v} - \frac{1}{u} \Rightarrow \frac{1}{f} = \frac{1}{v} - 0 \Rightarrow \frac{1}{f} = \frac{1}{-90} \Rightarrow f = -90 \qquad P = \frac{1}{f} = \frac{1}{-90} = -0.01D\]
Defects of Vision and Their Correction: Hypermetropia or farsightedness
Hypermetropia is a defect that affects the ability to see nearby objects while still being able to see distant objects clearly. The distant objects that can be seen are usually positioned more than 25 cm away from the eye. Hypermetropia can have two reasons:
- The first is low converging power. Here, the ciliary muscles are weakened and cannot thicken the eye lens when needed to increase its converging power. Hence, the image of close objects is formed behind the retina instead of on the retina.
- The other factor is the shape of the eyeball. When it is short, the retina is closer to the eye lens, causing the image of near objects to form behind the retina. See figure 4, where the focal point is clearly behind the retina.
Fig. 4 - Diagram of hypermetropia vision
Hypermetropia correction
By using convex lenses in front of the eye, the light rays from near objects converge through the lens, forming an image of the object close to the near point N of the eye. Depending on the power of the lens used, the converging power will be increased accordingly. Figure 5 shows a diagram of the corrected hypermetropic eye using convex lenses.
When utilising convex lenses, the image is created on the retina instead of behind the retina, as was shown in figure 3. N is the normal near point of the eye, which is 25 cm, while N' is the near point of a hypermetropic eye, which is more than 25 cm.
Fig. 5 - Hypermetropia correction
The power of the lens is set so that its focal length is equal to the near point of the eye. The required focal length or power needed to correct hypermetropia is calculated mathematically using the same formula given above.
However, for hypermetropia correction, u represents the object distance taken as the normal near point of the eye, while v is the near point distance.
Defects of Vision and Their Correction: Astigmatism
Astigmatism is a common defect of vision caused by an uneven or asymmetric retina or focus of the eye or an irregular shape of the cornea of the eye. This causes some images to be sharper than others, as the rays that reflect from the object approach different points in the irregular or uneven eye.
Astigmatism correction
Astigmatism can be corrected partially with cylindrical spectacles that oppose the irregularity of the eye. This means that if the refractive error of the eye is +1.5, the correction of the refractive error is -1.5. However, as the spectacles are fixed and the eyes move, the distance is always varying so that the vision is not totally corrected at all times.
This problem is eliminated using contact lenses that are fixed on the eye. As they cover a large portion of the irregular cornea, they provide a total correction. Another option is laser correction, which uses laser technology to reshape an irregular cornea. This can also be used for other defects besides astigmatism.
Defects of Vision and Their Correction: Cataract
A cataract is a condition in which the vision becomes very blurry, almost cloud-like. It develops when a membrane is formed over the eye lens (see figure 6), which causes the vision to deteriorate over time. This condition may even lead to vision loss if left untreated. It can be treated surgically by removing the formed cloudy membrane and replacing it with an artificial lens.
Fig. 6 - An eye with a cataract, showing the membrane over the eye. Source: Rakesh Ahuja MD for creative commons.
Defects of Vision and Their Correction: Presbyopia
Presbyopia, which happens gradually as we age, is caused mainly in older people due to weakened ciliary muscles of the eye. The muscles lose their flexibility and thus are unable to focus properly on near objects. This defect can happen in conjunction with myopia, astigmatism, or hypermetropia. It affects the ability to see nearby objects.
Presbyopia correction
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)
\[D = \frac{1000}{f[mm]}\]
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.
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