The human eye in many ways is similar to a camera. Like the camera, the eye lenses too focus images on the retina. The eye has two lenses: the cornea and “the lens.” The cornea, which is the first element in the visual pathway, provides about 2/3rd of the focusing power of the eye.

Sometimes in some unfortunate persons, there is a mismatch between the focusing power of the cornea and the lens and the length of the eye. In these circumstances, the eye focuses images in front of or behind the retina. This results in unclear vision, eyestrain, and headaches. The person is said to have a refractive error and needs to wear spectacles or contact lenses for clear vision.