The CORNEA is a transparent tissue in the front part of the eye. It is a curved spherical structure that is responsible for focusing the light onto the inside of the eye. The PUPIL is the black spot in the center of the iris. It is the opening in the iris through which light passes. The IRIS is the colored part of the eye. It opens up and constricts, depending upon how much light is present, to change the pupil size in order to control the amount of light entering the eye. The LENS is responsible for accommodation or helping to fine adjust the focus of the eye. The lens changes shape to focus on objects at near and distance for clear vision. The RETINA is a thin film of tissue (like film in a camera) where images are brought into focus. The retina lines the inside surface of the eyeball and is connected to the brain where the visual signals are processed.

The process of vision is complex. The eye receives light waves which are focused on the retina (the rear lining of the eye) and transmitted from there to the brain. The brain, in turn, creates a single image from the separate images it has received from the two eyes; this is known as fusion. To develop normal, binocular (two-eyed) vision, a child needs two healthy, well-coordinated eyes with relatively equal vision in each so that the images that form on the retina will have comparable clarity. If the muscles that move each eyeball have developed correctly, the eyes will work together as a team.

Visual acuity is a measurement of the clarity or sharpness of vision measured at a specific distance. The commonly used term "20/20 vision" describes clear vision at the distance of 20 feet. That is, a person with 20/20 vision can see clearly what a person with normal vision should see at that distance. If a person has 20/50 vision, it means that they must be as close as 20 feet to see what a person with normal vision could see at 50 feet.

A common misconception is that 20/20 indicates that a person has perfect vision. This only describes the person's ability to see with clarity and sharpness. There are many other important visual skills, including peripheral awareness or side vision, eye coordination, depth perception, accomodation or focusing ability, and color vision that contribute to overall visual ability.