The total optical power of the relaxed eye in humans is around 60 dioptres. The cornea is responsible for around two-thirds of this refractive power (about 40 dioptres), while the crystalline lens is responsible for the remaining one-third (about 20 dioptres). Thus, the eye can focus within a distance of about 200 mm from its surface. The human eye has the ability to change the shape of its lens to adjust its focal length. This mechanism is called "accommodation". Without accommodation, objects far away would appear blurry because their images would be focused on different parts of the retina at different times. Accommodation occurs when the ciliary muscle contracts to pull on the lens fibers, changing their shape and thereby altering their refractive index. This changes the curvature of the lens, causing it to become more or less distant from the retina. There are two types of accommodation: near vision accommodation and far vision accommodation. Near vision accommodation is needed whenever you read or work close up. It makes objects closer to you bigger and brighter. Far vision accommodation is needed when you look out at the world from a distance. It makes objects farther away smaller and darker. Without these adjustments made by the eye every day, long-distance viewing would be impossible because everything would be too blurry to see.