The light rays, which enter the eyeball from the visual field, are focused to ensure acute vision. The majority of this focusing is accomplished by the permanently rounded cornea.
Fine adjustments of focusing, for acuteness of vision, are provided by the crystalline lens (biconvex lens). This is particularly important when changing one's gaze between far and near objects.
The additional focusing provided by the crystalline lens, mentioned above, is one of the processes involved in accommodation. Accommodation refers to the various adjustments made by the eye to see better at different distances.
The crystalline lens is kept in a flattened condition by the tension of the zonular fibers (zonule ligaments; fibers of the ciliary zonule) around its equator, or margin. Contraction of the ciliary muscle of the eyeball releases this tension and allows the elastic lens to become more rounded.
Since the elasticity of the crystalline lens decreases with age, old people may find it very difficult to look at close objects. A second process in accommodation is the constriction of the pupils. The diameter of the pupil (the hole in the middle of the iris) controls the amount of light that enters the eyeball.
As a light source comes closer and closer, the intensity of the light increases greatly. Therefore, the pupils must be constricted to control the amount of light entering the eyeball as an object under view comes close to the individual.
A third process in accommodation is the convergence of the axes of the two eyeballs toward the midline. Since both eyes tend to focus on the same object (binocular vision), there is an angle between the two axes.
As an object draws closer, the angle increases to enable the axes to still intersect the object.