Accommodation is the way the eye increases optical power (the degree to which the lens converges or diverges light). This is necessary to produce a clear image (focus) on an object when it draws near the eye. A lens that is more convex( fatter in the middle) would refract more light rays than a less convex lens (thinner lens). The lens can change shape because the cells of the lens contain an elastic crystalline protein. The young human eye can change focus from distance to seven centimeters from the eye in 350 milliseconds.
The eye focuses on a given object by changing the shape of the eye lens through accommodation. This is controlled by ciliary muscle,which surrounds the lens.