Ciliary muscles help us to focus near and distant objects. The ciliary muscles can change the thickness of the soft and flexible eye lens and hence its focal length which, in turn, change the converging power of the 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.
