The human eye relies on bending light to create a sharp image. For clear vision, light passes through the cornea and the lens and must land precisely on the retina, the light-sensitive tissue at the back of the eye. If the eye’s structure causes the light to focus elsewhere, the result is blurred vision, known as a refractive error. These vision problems are categorized by how they affect a person’s ability to see objects at different distances. The most common errors are nearsightedness and its direct counterpart.
Farsightedness: The Opposite of Nearsightedness
Farsightedness, medically termed hyperopia, is the opposite of nearsightedness. Individuals with this refractive error typically see distant objects clearly but experience blurring when looking at objects nearby. The mechanism behind this blurring relates to the eye’s focusing power. Instead of light rays converging on the retina, the focal point lands theoretically behind the retina.
This misdirection means the light rays have not yet fully converged by the time they hit the retina, resulting in an unclear image. While characterized by blurry near vision, severe hyperopia can make objects appear blurry at any distance. Hyperopia is less prevalent than myopia, affecting an estimated 5 to 10 percent of people.
The Mechanics of Nearsightedness
Nearsightedness, or myopia, is the most common refractive error globally, contrasting with farsightedness. People with myopia see close objects clearly, but distant things appear blurry. The name refers to the clarity of “near” vision.
This occurs because the eye’s focusing system is too powerful, causing incoming light to converge too soon. The light rays focus in front of the retina, instead of directly upon its surface. By the time the light reaches the retina, the image is already out of focus, leading to a blurry perception of far-away objects.
Anatomical Differences in Vision Errors
The underlying cause of both nearsightedness and farsightedness is a mismatch between the eye’s length and its focusing power. In typical vision, the length of the eyeball and the curvature of the cornea work in harmony to ensure light focuses precisely on the retina. Refractive errors occur when the physical structure of the eye differs from the norm.
Causes of Farsightedness (Hyperopia)
Hyperopia is most often caused by an eyeball that is too short from front to back, a condition called axial shortening. Because the retina is too close, the focal point overshoots the target. Hyperopia can also be caused by a cornea that is too flat, meaning it does not bend light enough.
Causes of Nearsightedness (Myopia)
Myopia is typically a result of the eyeball being too long, a condition known as axial myopia. This elongated shape causes the light to converge prematurely, landing in front of the retina. The other primary cause of myopia is a cornea that is too steeply curved, which over-bends the light and forces the focal point forward.
Correcting Vision: Lenses and Procedures
Corrective measures for both hyperopia and myopia compensate for structural imperfections to ensure light focuses correctly on the retina. To treat farsightedness, focusing power must be added to push the focal point forward onto the retina. This is achieved using a convex lens, which is thicker in the center and thinner at the edges, thereby converging the light rays. Prescriptions for hyperopia are indicated by a positive (+) power.
Correcting nearsightedness requires reducing the eye’s excessive focusing power to pull the focal point back. This adjustment is accomplished with a concave lens, which is thinner in the center and thicker at the edges, causing the light rays to diverge. Prescriptions for myopia use a negative (-) power notation.
For a more permanent solution, refractive surgery, such as LASIK, can reshape the cornea to alter its curvature. For myopia, the procedure flattens the cornea to reduce focusing power; for hyperopia, the cornea is steepened to increase power.