The eye works like a camera. The major difference is that the camera has a layer of film in the back, while the eye has a layer of nerve tissue in the back, called the retina. The retina converts images into electrical impulses and sends it to the brain via the optic nerve. In the camera the lens focuses the light rays in order to form an image on the film. In the eye, two structures share this responsibility, the cornea and the lens. The cornea is like the window into the eye. It is a clear, transparent layer that lets light enter the eye through the pupil.

In the normal eye, light rays of an image pass through the cornea and the lens and are focused directly on the retina to produce a clear image. In a myopic (nearsighted) eye, the light rays pass through the cornea and lens but the point at which they converge (focus) is in front of the retina. This configuration allows clear images of near objects, but not those that are far away. When myopes are young, they can use their glasses for far and near tasks since their lens accommodates for near vision. After approximately 40 years of age, the human lens looses its elasticity so they frequently have to remove their glasses while reading or use bifocals.

In a hyperopic (farsighted) eye, the light rays do not converge or focus by the time they reach the retina. Younger hyperopic patients can focus on more distant objects and images that are close. As hyperopic patients age they lose the ability to accommodate and then have difficulty seeing distant and near objects. They will often require glasses for both distance and near (bifocals) in their forties.