At its most basic, refractive errors happen when the shape of your eyeball, cornea, or lens focuses light in front of or at the back of the retina, instead of on it. The cornea is the clear covering at the front of your eyes. It takes up most of the eye’s light-focusing job. The crystalline lens, which hardens in old age, flexes so you can easily see far away or up close. The retina is the light-sensitive surface at the back of your eyes. Its photoreceptors convert light waves into electrical signals. These signals are sent to your brain via the optic nerve. The brain then interprets these signals to form images.