Accommodation refers to the ability of the crystalline lens (located behind the pupil) of the eye to instantly change shape in order to bring into focus objects located at different distances from our face. For example, we might need to focus on the moon (infinity), our computer (intermediate, or about 60 cm) or a book (near, or about 40 cm). Different objects located at varying distances require our crystalline lens to quickly adjust to see them clearly. This process is similar to focusing a camera lens on an object when taking a picture.
There is a natural, but inevitable loss in the focusing power of our crystalline lens as we age, called Presbyopia. The ability of the crystalline lens to instantly change shape in order to focus light rays on the retina for near vision tasks gradually declines throughout life. Around age 43, most people begin to notice they must hold reading material farther away than usual. To correct this problem, bifocals, reading glasses or monovision contact lenses are increasingly required so by age 50 there is generally total dependence on corrective lenses.