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Visual Acuity: What is 20/20 Vision? 

Introduction
Visual acuity refers to the smallest detail that you are able to see.  Visual acuity testing, such as reading letters or shapes on a chart, measures the eye’s potential central vision or detailed vision.  Visual acuity is a measure of the sharpness of sight, not overall vision quality.  Visual acuity scores are written as a fraction.  For example, 20/20 vision means that a person can see details that should “normally” be seen at a distance of 20 feet while 20/40 would mean that what a normal person sees at 40 feet you see at 20 feet.

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Anatomy
Your eyes and brain work together with amazing efficiency.  Light rays enter the front of your eye and are interpreted by your brain as images.  Light rays first enter your eye through the cornea, the “window” of your eye.  The cornea is a clear dome that helps your eyes focus.
 
The anterior chamber is located behind the cornea and in front of the iris.  The anterior chamber is filled with  a fluid that maintains eye pressure, nourishes the eye, and keeps it healthy.  The iris is the colored part of your eye.  Eye color varies from person to person and includes shades of blue, green, brown, and hazel.  The iris contains two sets of muscles.  The muscles work to make the pupil of your eye larger or smaller.  The pupil is the black circle in the center of your iris.  It changes size to allow more or less light to enter your eye.
 
After light comes through the pupil, it enters the lens.  The lens is a clear curved disc.  Muscles adjust the curve in the lens to focus clear images on the retina.  The retina is the back part of your eye.
 
Your inner eye, or the space between the posterior chamber behind the lens and the retina, is the vitreous body.  It is filled with a clear gel substance that gives the eye its shape.  Light rays pass through the gel on their way from the lens to the retina.
The retina is a thin tissue layer that contains millions of nerve cells.  The nerve cells are sensitive to light.  Cones and rods are specialized receptor cells.  Cones are specialized  for color vision and detailed vision, such as for reading or identifying distant objects.  Cones work best with bright light.  The greatest concentration of cones is found in the macula and fovea at the center of the retina.  The macula is the center of visual attention.  The fovea is the site of visual acuity or best visual sharpness.  Rods are located throughout the rest of the retina.
 
Your eyes contain more rods than cones.  Rods work best in low light.  Rods perceive blacks, whites, and grays, but not colors.  They detect general shapes.  Rods are used for night vision and peripheral vision.  High concentrations of rods at the outer portions of your retina act as motion detectors in your peripheral or side vision. 
 
The receptor cells in the retina send nerve messages about what you see to the optic nerve.  The optic nerves extend from the back of each eye and join together in the brain at the optic chiasm.  From the optic chiasm, the nerve signals travel along two optic tracts in the brain and eventually to the occipital cortex, where you process and perceive vision.

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Diagnosis
Uses
 
Visual acuity is a measure of your detailed sharp vision.  It is an indication of the smallest detail that you are able to see.  Visual acuity testing is usually performed with and without glasses or contacts to record the best visual acuity with and without lens correction.
 
Preparation
 

There is no preparation for visual acuity testing.
 
Testing and Results
 
Visual acuity testing is simple.  Your eye doctor will use a standardized letter or shape chart.  You will be asked to state the smallest item that you can see.  Your eyes will be tested individually and together.  You will be tested with and without your glasses or contact lenses, if you wear them.

Visual acuity scores are written as a fraction for each eye.  For example, 20/20 vision means that your eye sees the detail that they should see from a distance of 20 feet.  A score of 20/40 means that the eye can only see detail at twenty feet that it should be able to see at 40 feet.  The higher the second number, the worse the visual acuity.

Glasses or contact lenses can correct visual acuity.  A score of 20/200 with the best corrective lenses possible is considered legally blind.  Conversely, some people may have better than 20/20 vision.  For example, a score of 20/15 means that an individual can see at 20 feet detail that “normally” is seen at 15 feet. 
 
Keep in mind that 20/20 vision is considered a testing norm for detailed vision only.  It is useful for monitoring vision changes and for testing corrective lenses.  It is not a measurement of overall eye health or functioning.

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This information is intended for educational and informational purposes only. It should not be used in place of an individual consultation or examination or replace the advice of your health care professional and should not be relied upon to determine diagnosis or course of treatment.

The iHealthSpot patient education library was written collaboratively by the iHealthSpot editorial team which includes Senior Medical Authors Dr. Mary Car-Blanchard, OTD/OTR/L and Valerie K. Clark, and the following editorial advisors: Steve Meadows, MD, Ernie F. Soto, DDS, Ronald J. Glatzer, MD, Jonathan Rosenberg, MD, Christopher M. Nolte, MD, David Applebaum, MD, Jonathan M. Tarrash, MD, and Paula Soto, RN/BSN. This content complies with the HONcode standard for trustworthy health information. The library commenced development on September 1, 2005 with the latest update/addition on April 13th, 2016. For information on iHealthSpot’s other services including medical website design, visit www.iHealthSpot.com.