Good vision requires a sharp image being produced on the retina at the back of the eye. The optical system (cornea, crystalline lens and vitreous body) of a normal eye diverts light rays in a way that they land exactly in focus at the point of best vision on the retina, the so-called makula. This diversion of light rays is called refraction. The total refractive power is expressed in diopters (D). Total refractive power is determined on the basis of the different parts of the optical system. When the total refraction is zero diopters, objects will be depicted sharply on the retina.

An elongated eyeball (eye that is too long from front to back) is typically the cause of nearsightedness. Because the eye is longer than it should be, the incoming light rays tend to focus in front of the retina, instead of on it. This causes distant objects to appear blurry. The degree of nearsightedness determines the distance that a patient can see. A patient with severe myopia may only be able to see objects clearly up to a few inches away, while a patient with mild myopia may be able to focus on and see objects clearly several yards away. The optical correction of nearsightedness decreases the refractive power of the optical system,by moving the focal point forwards onto the retina.