The following are the most common refractive errors, all of which affect vision and may require corrective lenses:
1. Myopia, or Nearsightedness
Myopia, or nearsightedness, is the result of refractive power that is too strong in relation to the eye's length. This can be from a cornea that is too curved, or from an eye that is too long. Because of this, the images of distant objects are focused in front of the retina, and are blurred when they reach the retina, causing the affected person to see more clearly up close than in the distance. Approximately 20% of the Thai population is nearsighted.
Near-sightedness or Myopia
2. Far-sightedness, Hypermetropia or Hyperopia
Hyperopia, also known as hypermetropia or farsightedness (inborn type), is the result of refractive power that is too weak in relation to the eye's length, from a cornea that is too flat, or from an eye that is too short. As a result, the optical system is not able to focus the images before the light rays reach the retina, which then only receives a blurred image.
Vision in the distance may or may not be clear, and closer objects do not come into proper focus. Your eyes can compensate for a small amount of hyperopia to see distance clearly, but as you get older or and more focusing is required, you may experience symptoms of headaches or eye fatigue and lose this compensating ability, giving blurry vision at both distance and near.
Far-sightedness, Hypermetropia or Hyperopia
3. Astigmatism
Astigmatism is the result of the general inability of the eye to clearly focus images from any distance, due to uneven curvatures of the cornea. Instead of being spherical in shape like a basketball, the cornea shape is more similar to that of an egg or rugby ball. Light is projected onto the retina by the cornea, and if the cornea is uneven, then the image perceived by the retina will not be clear. This focusing problem often exists concurrently with nearsightedness, or inborn farsightedness. It is correctable by glasses, contact lenses, or refractive surgery.
Astigmatism
4. Presbyopia
As children and young adults, we often take our vision for granted. We can look at something at a distance, and then read the finest print in the newspaper without giving it a second thought. Then around the ages of late 30s and early 40s, we start to notice that we need better lighting for small print or that our arms aren’t long enough anymore and that things really close up are hard to see. Presbyopia is a general condition affecting those over 38 years of age, whereby the lens and eye muscles used in focusing begin to lose the ability to change shape to focus on near objects. This condition is often confused with inborn farsightedness (or hyperopia).
The natural lens (crystalline lens) inside the eye is responsible for changing our focus from distance to near. When objects are at different distances from us, the light rays coming into the eye fall onto the retina differently. The crystalline lens brings things at different distance into focus by changing its shape. This is an automatic process, there is no thinking or conscious effort involved. As we get older, this lens loses its flexibility and cannot change shape like it used to. There is no cure for presbyopia, and no one can escape it, it is a normal change due to the aging process.
Presbyopia
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