Normal sight relies on healthy eyes, and normal connections between the eye and brain. The eye's job is to focus a clear image onto a film (retina) at the back of your eye (much like a camera). Many receptors in the retina convert light into electrical signals which are transported to that part of the brain responsible for sight. The macula is the central area of the retina which is responsible for our sharp central sight.
The eye has two focusing systems which are cornea and lens (whereas the camera has only one focusing system). The cornea is the transparent coat in front of your eye's colored part (iris). The lens is that piece found behind the iris. The pupil is a hole in the center of the iris. This pupil can change its size and thus control the amount of light entering the eye.
The lens is enveloped by a bag (capsule). The material within this capsule is soft and clear in children but it gradually hardens and yellows with age. The entire lens is suspended in place within the eye by threads called (zonules).
These zonules are connected to muscles which, with contraction, can change the shape of the lens, allowing a wide range of focusing (for near and far distances) in young people. When becoming older, the lens looses its flexibility, leading to difficulty seeing near objects. This usually starts around age 40 years, and the patient requires (reading glasses) to clearly see near objects.