A normal eye can view objects at various distances by the power of accomodation . The power of accomodation is the capability of the lens to change its focal length so that light from objects at varying distances will fall on the retina itself.
For example: When light comes from a distant object , the ciliary muscles loosen their hold on our eye lens and the lens become thin. This change in the lens's aperture inceases the focal length allowing light to fall on the retina. When light falls on the eye from short distances , the ciliary muscles constrain , making the lens become smaller and rounder . This causes the focal length to become small and the light is focussed on the retina.