Normal Vision
Before a light image is focused on the retina, it is refracted three times: by the cornea and when it both enters and exits the lens. The cornea is actually more refractive than the lens; however the it is not capable of adjusting its width to allow the object to become focused while the lens is. Our eyes are best suited for viewing objects from a distance. In order to focus on distant objects, all one has to do is aim both eyes at the same spot. The minimum distance where no adjustment of the lens is required to focus is called the far point of vision.
Focusing on objects closer than 6 m from the eye is more complicated. Light diverges as it nears the eye, coming to a focal point farther from the lens. Three adjustments of the eye are required for close vision: 1) Accommodation of the lens, 2) Constriction of the pupils, and 3) Convergence of the eyes. Accommodation of the lens is when the ciliary muscles contract to cause the lens to bulge. This increases the refractory ability of the lens, bending the light more sharply. The lens is at its maximum bulge at the near point of vision, the closest distance from the eye an object can be focused on. The normal near point of vision for young adults is about 10 cm from the eye. Constriction of the pupil reduces the size of the opening, preventing divergent rays from entering the eye. Convergence of the eyeballs is the rotation of the eyes to direct them towards the object being viewed.
