To see objects at different distances the eye changes its power – just like a camera lens. In the eye this is achieved by a structure called the crystalline lens. When we are young, the lens is like a sack full of jelly. The lens is attached by very fine fibres to a circular muscle. When this muscle constricts, the lens bulges out and close objects come into focus. As we get older the lens continues to grow and becomes inflexible and the closest point that we can focus on gradually gets further away. Most people over the age of 45 require glasses for reading.