More than 25% of people in the world have some degree of myopia (approximately 15 million of Iranians are nearsighted). Myopia is a medical term for nearsightedness. The most important cause of myopia is genetic factors. Myopia usually tends to start in the early second decade of life and increases as the eyes grow in length during puberty, and usually stops after age 18. It has not been proven that reading at an early age, prolonged reading, reading in the dark, watching television too closely, or wearing inappropriate glasses would cause myopia.
Myopia occurs when the curvature of the cornea is too steep or the size of the eye is more than normal in relation to the corneal curvature. As a result, the light in the eye does not focus accurately on the retina and the resolution of the images decreases. The term nearsightedness means you can clearly see close objects but distance objects are blurry. Of course,close objects, viewed at the proper distance, can be seen clearly because the focus of their light rays matches the refractive error of the nearsighted eye. For example, a patient with -2 diopter correction is naturally in focus at 50 centimeters. People with less than -3 diopter correction can clearly see objects without glasses. Many people with myopia do not need to wear glasses when reading.