Distant: The parallel rays of light coming from distant tree needs a convex eye-lens of low converging power to focus then to form an image on the retina. When looking at the distant tree, 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.
Nearby: The diverging rays of light coming from book needs a convex eye-lens of high converging power to focus then to form an image 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.
