The eye relies on the cornea and lens to form an image. The cornea is the transparent, dome-shaped front surface of the eye. The lens, in turn, refracts incoming light, focusing it onto the retina at the back of the eye. Unlike the cornea, the lens is flexible and can change shape with the help of the surrounding ciliary muscles. When looking at distant objects, these muscles relax. When focusing on nearby objects, the muscles contract, increasing the curvature of the flexible lens and altering its focusing power. Presbyopia results from the hardening of the eye's lens with age. As the lens loses its flexibility, it can no longer make the necessary shape changes to focus on near images.