The human eye is a complex mechanism in many ways similar to a camera. Just as the camera records images on a film, the eye records them on a special layer called the retina. Just like the camera needs lenses to focus images on the film, the eye also has lenses to focus images on the retina. The eye has two lenses: the cornea and the “lens”. The cornea is a fixed focus lens. It simply bends light rays by a fixed amount. The lens is like an auto focus system. It changes its shape to focus images perfectly on the retina, so that images appear sharp and crisp.

Sometimes when there is a mismatch between the curvature of the cornea, the power of the lens and the length of the eye, the eye focuses images ahead of or behind the retina. This is known as a refractive error. This results in unclear vision, eyestrain and headaches. To eliminate this mismatch, spectacles or contact lenses need to be worn.