Most of that refraction in the eye takes place at the first surface, since the transition from the air into the cornea is the largest change in index of refraction which the light experiences. About 80% of the refraction occurs in the cornea and about 20% in the inner crystalline lens.
While the inner lens is the smaller portion of the refraction, it is the total source of the ability to accommodate the focus of the eye for the viewing of close objects. For the normal eye, the inner lens can change the total focal length of the eye by 7-8%. Common eye defects are often called refractive errors and they can usually be corrected by relatively simple compensating lenses.
Light that passes through the pupil opening, will enter the crystalline lens. The crystalline lens is made of layers of a fibrous material that has an index of refraction of roughly 1.40. Unlike the lens on a camera, the lens of the eye is able to change its shape and thus serves to fine-tune the vision process. The lens is attached to the ciliary muscles. These muscles relax and contract in order to change the shape of the lens. By carefully adjusting the lenses shape, the ciliary muscles assist the eye in the critical task of producing an image on the back of the eyeball.