The lens is the clear part of the eye that helps to focus light, or an image, on the retina. The retina is the light-sensitive tissue at the back of the eye. In a normal eye, light passes through the transparent lens to the retina. Once it reaches it, light is changed into nerve signals that are sent to the brain. In order for the retina to receive a sharp image, the lens must be clear. But if the lens is cloudy from a cataract, the resulting image will be blurry.