There are four main categories of refractive error:
Myopia Nearsightedness, or myopia, as it is medically termed, is a vision condition in which close objects are seen clearly but objects farther away appear blurred. Nearsightedness occurs if the eyeball is too long or if the cornea, the clear front cover of the eye, has too much curvature. As a result, the light entering the eye isn’t focused correctly and distant objects look blurred.
Nearsightedness is a very common vision condition affecting nearly 30 percent of the population. Some research supports the theory that nearsightedness is hereditary. There is also growing evidence that it is influenced by the visual stress of too much close work.
Generally, nearsightedness first occurs in school-age children. Because the eye continues to grow during childhood, it typically progresses until about age 20. However, nearsightedness may also develop in adults due to visual stress or health conditions such as diabetes.
Hyperopia Farsightedness, or hyperopia, as it is medically termed, is a vision condition in which distant objects are usually seen clearly but close ones do not come into proper focus. Farsightedness occurs if your eyeball is too short or if the cornea has too little curvature, so light entering your eye is not focused correctly.
Common signs of farsightedness include difficulty in concentrating and maintaining a clear focus on near objects, eye strain, fatigue and/or headaches after close work, aching or burning eyes, irritability or nervousness after sustained concentration.
Common vision screenings, often done in schools, are generally ineffective in detecting farsightedness. A comprehensive optometric examination will include testing for farsightedness.
In mild cases of farsightedness, your eyes may be able to compensate without corrective lenses. In other cases, your optometrist can prescribe eyeglasses or contact lenses to optically correct farsightedness by altering the way the light enters your eyes.
Astigmatism is simply a non-spherical curvature of the cornea or lens. It’s much like a cylinder where one radius is different from the other, and so to correct this, we create a lens to counteract that curvature and hence render an optically perfect spherical system.
Presbyopia usually begins around the age of 45. Up until about this age, both the pulling action of the muscles and the elasticity of the lens allow the lens to focus on objects up close and at a distance. Eventually, the lens loses its elasticity and hardens, making it difficult to focus on near objects. Simultaneously, the muscles that adhere to the lens weaken and the end result is blurring vision of near objects. Hence, reading glasses or magnifiers need to be worn. People who are nearsighted will still undergo presbyopia when they are wearing their distance glasses, and so in order to see up-close, they need to remove their glasses and use their natural “near-sighted” ability.