So how do those corrective lenses work? The short answer is that they compensate for the loss of refractive accuracy in your eyes. The full answer is a larger discussion.
The human eye is formed by an outgrowth of neural tissue early in human development within the womb. Starting from the posterior side, the rearmost portion of each human eye is connected to the optic nerve, which travels into the brain and interfaces with the visual cortex. The retina converts incoming light into electrical signals that are carried by the optic nerve to the visual cortex where they are processed to give rise to perception.
At the anterior side of the eye, the surface anatomical structures serve to focus light into the retina. The cornea is a transparent layer of tissue that light passes through in order to enter the pupil. The concave shape of the cornea refracts (bends) light to redirect it inwards. Directly behind the pupil is a lens, which further refracts light rays to focus them onto the retina. The muscles of the iris can expand or contract to change the size of the pupil and adjust for the amount of light that enters. It functions like the shutter on a camera, adjusting for brightness and glare.
While the cornea has more refractive power than the lens behind the pupil, its anatomy is static. It does not change shape. However, the lenses behind our pupils can expand or contract to alter the strength of refraction. In order for an image to appear clearly, the light rays from the entire area of the pupil must be refracted precisely to the area of the retina. By bending light rays, we are able to change the point within our eye at which light rays converge, thus shifting our focal point from near to far and vice versa.
Life in focus
Everyone has a point beyond which the lens cannot adequately adjust to focus an image. There is a certain distance beyond which you cannot make out the letters of a sign and another distance at which even the borders of your finger will blur directly in front of your eye. These distances are referred to as the far point and near point, respectively, and vary significantly between individuals. When either point deviates too much from the average, it can become difficult to adequately perceive images.
Myopia and hyperopia
In myopia or “nearsightedness,” the lens behind the eye bends light too much, either because of the structure of the lens or the eyeball itself. Individuals with myopia often have elongated eyeballs. In either instance, light rays entering the pupil converge at a point in front of the retina. This causes blurry vision. When an object is close to the eye, reflected light rays naturally diverge (bend outwards) more than at a distance, and this compensates for the refractive error of the lens in an individual with myopia. Images thus appear clearly in focus when they are close enough to the eyes.
In hyperopia or “farsightedness,” the lens does not sufficiently refract light rays towards the retina. As a result, the light rays converge behind the retina, again resulting in blurry vision. In the inverse of individuals with nearsightedness, who are able to make out objects close to the eye, individuals with farsightedness are able to make out images at a distance because the light naturally diverges less.
The treatment for myopia or hyperopia is straightforward. An optometrist tests for the refractive error in the lens and crafts an artificial one to compensate for gain or loss. However, artificial lenses are static structures similar to the cornea. They cannot adjust to the distance of our focal point, especially in the absence of signals from the brain. Corrective eyewear for nearsightedness will affect vision at a distance and the inverse is true of eyewear for farsightedness. For individuals with severe vision impairment, bifocals may be prescribed in which two lenses are present, one focusing light at a distance and one focusing light from nearby objects.