Nearsightedness and farsightedness are common vision conditions that affect how clearly a person sees the world at different distances. Medically, these are known as refractive errors, meaning the eye does not bend or refract light correctly to form a clear image. When light enters the eye, it must be focused precisely onto a specific point. Any deviation from this process leads to blurry vision.
How the Eye Focuses Light
To achieve clear vision, the eye must accurately bend light rays that enter through the pupil. This process, called refraction, is primarily handled by the dome-shaped cornea and the crystalline lens situated behind the iris. The cornea provides the majority of the eye’s focusing power. The lens fine-tunes the focus for objects at various distances, a process called accommodation.
In an eye with normal vision, known as emmetropia, the light-bending components work in harmony with the eye’s length. The cornea and lens ensure that incoming light rays converge into a single, sharp focal point directly on the retina. The retina then converts the light into electrical signals sent to the brain. When the eye’s shape deviates from this ideal configuration, the focal point shifts, resulting in a refractive error.
The Mechanics of Nearsightedness
Nearsightedness, or myopia, occurs when the eye focuses light too strongly, causing the image to form in front of the retina. People with this condition typically see objects up close clearly, but distant objects appear blurred. This focusing error often happens because the eyeball has grown slightly too long from front to back (axial myopia).
Another common cause is a cornea that is curved too steeply, possessing excessive focusing power. In either situation, the light rays converge before they reach the retina’s surface. By the time the light hits the retina, the rays have begun to spread out again, creating a fuzzy image of distant objects.
The Mechanics of Farsightedness
Farsightedness, or hyperopia, is the opposite refractive error, where the eye does not bend light strongly enough, causing the focal point to land behind the retina. Individuals often see distant objects clearly, but they struggle with objects that are near. This problem typically arises because the eyeball is slightly too short, meaning the distance between the cornea and the retina is insufficient for the light to converge properly.
The condition can also be caused by a cornea or lens system that is too flat, lacking the required refractive power. The eye attempts to compensate for this focusing deficit by using the lens’s accommodation mechanism. This compensation can lead to eye strain and headaches, especially during close-up tasks. For younger people with mild hyperopia, the eye’s natural ability to accommodate may allow for clear vision, but this ability lessens with age.
Correcting Refractive Errors
Correcting these errors involves introducing an external lens to change the path of light before it enters the eye, ensuring the final focal point lands precisely on the retina. For nearsightedness, a concave lens is used, which is thinner in the center and thicker at the edges. This lens works to diverge the light rays slightly, effectively pushing the focal point backward onto the retina.
Farsightedness is corrected using a convex lens, which is thicker in the center and thinner at the edges. This lens adds focusing power to the eye’s natural system, causing the light rays to converge more strongly. These corrective lenses are available as eyeglasses or contact lenses.
Refractive surgery offers a more permanent solution by physically altering the shape of the cornea itself. Procedures like LASIK or PRK use a precise laser to remove minute amounts of corneal tissue. For myopia, the center of the cornea is flattened to reduce its focusing power. For hyperopia, the sides of the cornea are reshaped to make the center steeper, increasing its focusing power. This permanent reshaping adjusts the eye’s primary refractive surface, allowing light to focus correctly.