It protects the pupil, iris and the inside of the eye from the penetration of foreign bodies and is the first and most powerful element in the focusing system of the eye. As light passes through the cornea, it is partially refracted before it reaches the lens. The curvature of the cornea, which is spherical in childhood but changes with age, gives it the ability to focus; when the curve becomes irregular, it causes a focus defect called astigmatism, which makes images appear elongated or distorted.