Why do some people need glasses?
Our eyes are one of the most sensitive organs in our bodies. They focus light on the retina which has photoreceptors (rods and cones) that convert light stimuli into chemical signals that our brains can interpret. However, because of that sensitivity, sometimes our eyes are not able to focus light on our retinas properly. This may be caused by a plethora of reasons (injury, genetics, bad habits, illness, etc.). Since so many factors play a role in the quality of an individual's vision, over 75% of people in the US have to wear corrective glasses (TVC, 2021). There are many treatments for vision correction, (eg. contact lenses, LASIK and Femto LASIK), but this article will explain how glasses can be used to correct three main vision problems: Hypermetropia, Myopia, and Astigmatism.
Hypermetropia, also known as Long-sightedness or Hyperopia as shown in the diagram, is when the cornea is either too flat or the eye is compressed, causing a refractive error and causing light to focus too far back in the eye. This leads to light rays from close objects not focussing properly on the retina. In other words, anything that is close to the eye (eg. a book) is blurry.
Hypermetropia can be corrected through a convex lens. This is a lens that is thicker in the centre than on the edges. This enables the light to be refracted in a way such that a new focus point can be created that lands on the retina for a clear image. Convex lenses bring the focus point forwards so that light is correctly focused on the retina.
Myopia occurs when the cornea is too curved or the eye is elongated, once again leading to a refractive error, in which light is focused before it comes in contact with the retina. This means that objects close to the eyes are clear, but objects far away seem blurry. This condition is also known as Near-sightedness.
Myopia can be corrected through the use of a concave lens. This lens is thicker on the edges compared to the centre. Unlike the convex lens that focuses light, a concave lens diverges light. This is used to treat Myopia as it can cause the focus point of light to be pushed back, thus, hitting the retina and creating a clear image.
Astigmatism is when the lens or the cornea has abnormalities in its curvature. Normally, the lens and the cornea have an evenly rounded shape which helps focus light properly onto the retina. People with astigmatism have uneven curves, hence they may suffer from blurry vision when looking at both near and far objects. Furthermore, this condition can also develop in people who already have myopia or hypermetropia.
Astigmatism can be treated through a corrective cylindrical lens. This lens helps to refract light onto the retina correctly. With Astigmatism, often there are multiple focus points due to the uneven surface of the cornea or lens structure. Each lens is calibrated to match the exact nature of the patient's astigmatism and they counteract the effects of an uneven cornea and/ or lens. Some corrective cylindrical lenses can also correct hypermetropia and myopia in conjunction with astigmatism.
Interpreting a prescription
Every prescription is unique and tailored to fit the needs of the patient, therefore, it is important to fully comprehend the values associated with a prescription. OD is an abbreviation for oculus dexter (the right eye). OS stands for oculus sinister (the left eye). “Sphere” indicates the strength of the lens a patient is prescribed. A negative sign before the number indicates myopia and a concave lens is needed whereas a positive number indicates hypermetropia and a convex lens is needed for vision correction. “Cylinder” indicates the amount of lens power needed for astigmatism. “Axis” describes the lens needed to correct astigmatism. “Prism” is used to measure eye alignment problems (eg. lazy eye). “Add” stands for added magnifying power usually for older people who develop far-sightedness due to old age (presbyopia). All of these factors combined contribute to the type of lens and glasses prescribed to a patient.
Why it is essential to see an ophthalmologist regularly?
All conditions within this article are easily corrected and fairly common, however, there are other more severe eye conditions that can be irreversible and possibly life-threatening. Thus it is essential that one sees an ophthalmologist every 2-4 years. Without healthy vision, one's ability to work, drive, play sports or even recognize faces is compromised. Furthermore, other factors such as diabetes, blood pressure, and genetics all affect your eyes so it is important to get a complete medical eye exam to treat conditions effectively.
Written by: Amaan Aliani
Edited by: Samuel Lim 13L
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