How The Eye Works and 20/20 Vision


 

 

 

 

 

 

 

Our ability to "see" starts when light reflects off an object at which we are looking and enters the eye. As it enters the eye, the light is unfocused.

The first step in seeing is to focus the light rays onto the retina, which is the light sensitive layer covering the inside of the eye. If light is completely focused on the retina, we interpret the image as "clear." If the image is focused either before or after the retina, the image appears blurry to us. This is called a refractive error. 

What Pathway Does Light Take to Get to the Retina and Brain? 

Light entering the eye is first bent, or refracted, by the cornea -- the clear window on the outer front surface of the eyeball. The cornea provides most of the eye's optical power or light-bending ability. The amount of light entering the eye is controlled by the iris, the colored portion of the eye. After the light passes through the pupil, the hole in the iris, it is bent again -- to a more finely adjusted focus -- by the crystalline lens inside the eye. The lens focuses the light on the retina. This is achieved by the ciliary muscles and zonules in the eye changing the shape of the lens, bending or flattening it to focus the light rays on the retina.  Ideally, the light is "refracted" in such a manner that the rays are focused into a precise image on the retina. This allows us to see an image clearly. 

Even with the light focused on the retina, the process of seeing is not complete. For one thing, the image is inverted, or upside down. There are two special receptor cells in the retina, the rods and cones. Rods are mainly found in the peripheral retina and are primarily responsible for night vision and peripheral vision. Cones are principally found in the central retina and provide detailed vision for such tasks as reading or distinguishing distant objects. They also are necessary for color detection. 

Once the light is focused and the rods and cones are stimulated, millions of electrochemical impulses are sent along the optic nerve, eventually arriving at the visual cortex of the brain, located at the back of the head. Here, the electrochemical impulses are unscrambled and interpreted. The image is re-inverted so that we see the object the right way up. This "sensory" part of seeing is much more complex than the refractive part -- and therefore is much more difficult to influence accurately.
 

What is 20/20 Vision?

You may be pleased to hear that you have 20/20 vision and think you have perfect vision. But do you? Not necessarily.

 

20/20 only indicates how sharp or clear your vision is at a distance. Overall vision also includes peripheral awareness or side vision, eye coordination, depth perception, focusing ability, and color vision.

20/20 describes normal visual clarity or sharpness measured at a distance of 20 feet from an object. If you have 20/20 vision, you can see clearly at 20 feet what should normally be seen at that distance. If you have 20/100 vision, it means that you must be as close as 20 feet to see what a person with normal vision can see at 100 feet.

The ability to see objects clearly is affected by many factors. Nearsightedness, farsightedness, astigmatism, and/or eye diseases all influence visual acuity. Most people with vision slightly below 20/20 function very well, whereas some people who have better than 20/20 vision feel that their vision is not satisfactory. Everybody's visual expectations are different and satisfactory vision is far more complex than just being able to see 20/20.

A comprehensive eye examination by our optometrists will identify causes that may affect your ability to see well. They may be able to prescribe glasses, contact lenses, refractive surgery, or a vision therapy program that will help improve your vision. If the reduced vision is due to an eye disease, the use of ocular medication or other treatment may be needed.