For normal vision, light must be focused precisely onto the retina in back of the eye. This focusing is made by both the lens and the cornea.
The lens helps focus the image onto the retina in the back of the eye. Unlike the cornea, which has a fixed power, the focusing power of the lens can be adjusted. Ciliary muscles attached to the lens by small fibers, called zonules, can change the shape of the lens. This is what allows the eye to focus at various distances. (see below)
AGING – PREBYOPIA
Sometime between age 40 and 50, the lens begins to lose its flexibility. When this happens, the eye can no longer change its focus from distance to up-close vision. That is why people with normal vision before age 40 need reading glasses as they get older.