Ciliary muscles An eye can focus the images of distant objects and nearby objects on its retina by changing the focal length of its lens. The focal length of the eye lens is changed by the action of ciliary muscles. The ciliary muscles can change the thickness of the soft and flexible eye lens, and hence, the focal length, which in turn, changes the converging power of the eye lens.
When looking at the distant object, ciliary muscles are fully relaxed and the eye lens is very thin. Its focal length is maximum in this position and converging power is minimum to focus the parallel rays on the retina.
When looking at the nearby object, ciliary muscles get stretched and the eye lens becomes thick. Its focal length is minimum in this position and converging power is maximum to focus the diverging rays on the retina.