How Vision Works

The eye is a complex organ that works very similiar to that of a camera by focusing light rays and forming images. On the surface of the eye is the cornea, a thin, spherical layer of tissue that provides a clear window for light to pass through. In a normal healthy eye, the cornea bends or refracts light rays so that they focus precisely on the retina in the back of the eye. When light enters the eye it first passes through the cornea, then the aqueous humor, the crystalline lens, the vitreous humor and then ultimately it reaches the retina.

Located beneath the cornea is the iris, the colored part of the eye. In the center of the iris is the pupil. The iris functions like a shutter, adjusting pupil size to control the amount of light entering the eye.

Behind the iris is the crystalline lens, which works together with the cornea and the vitreous to focus light. Like a lens in a camera, the lens adjusts light rays as vision shifts between up close and distant objects in a process called accomodation.

Light is then dispersed through the vitreous, a gelatin substance that fills most of the eye and gives it its shape.

The back of the eye is lined with a thin layer of tissue containing millions of photoreceptor cells called rods and cones that are very light sensitive. The rods handle vision in low light while the cones handle color vision and detail. Once the light rays reach the retina, the light rays are focused into an inverted or upside down image. In the center of the retina is the macula. The macula is responsible for clear central vision. The retina converts the image into an electrical signal that travels down the optic nerve and to the brain.

            Normal Vision

Vision or visual acuity is tested by reading a Snellen eye chart at a distance of 20 feet. This chart consists of random letters of different sizes. The letters for normal vision (20/20) are 3/8 inches tall. If you have 20/20 vision, it means that when you stand 20 feet away from the chart you can see what a normal human being can see with no visual correction. In other words, if you have 20/20 vision your vision is normal -- a majority of people in the population can see what you can see at 20 feet.

If you have 20/40 vision, it means that when you stand 20 feet away from the chart you can only see what a normal human can see when standing 40 feet from the chart. That is, if there is a normal person standing 40 feet away from the chart, and you are standing only 20 feet away from the chart, you and the normal person can see the same detail. 20/100 means that when you stand 20 feet from the chart you can only see what a normal person standing 100 feet away can see. 20/200 is the cutoff for legal blindness in the United States.

You can also have vision that is better than the norm. A person with 20/10 vision can see at 20 feet what a normal person can see when standing 10 feet away from the chart.