Visual acuity is a medical term for the clarity and sharpness of your eyesight, measuring how well you distinguish shapes and details at a given distance. Eye care professionals use a visual acuity test to quantify this aspect of vision. This test is a primary step in identifying vision changes and detecting common refractive errors as part of a routine eye exam.
Common Types of Visual Acuity Charts
The most widely recognized tool for testing visual acuity is the Snellen chart. This chart features eleven rows of capitalized letters, starting with a single large letter at the top. The letters in each subsequent row become progressively smaller. While many are familiar with this classic wall chart, modern clinics may use video monitors to display the letters or images.
For individuals who cannot read letters, such as young children, alternative charts are used. The “Tumbling E” chart is a common substitute, which displays the capital letter “E” in various orientations. The person being tested is asked to indicate the direction the “E” is facing. Other charts employ simple pictures, like LEA symbols, for testing preschool-aged children.
How the Test Is Performed
You will be asked to sit or stand at a specific distance from the eye chart. The test is conducted on each eye individually to determine its specific capabilities. Your eye care specialist will ask you to cover one eye while you read the chart with the uncovered eye.
You will be instructed to read the smallest line of letters or symbols that you can see clearly. It is acceptable to guess if you are unsure of a letter, as this helps the examiner determine the limits of your vision. If vision is reduced, the test may be repeated while you look through a pinhole occluder. This device can help determine if the issue is a refractive error that can be corrected with glasses.
Understanding Your Visual Acuity Score
Your visual acuity is expressed as a fraction, such as 20/20. This notation is not a percentage but a specific measurement based on testing standards. The top number of the fraction represents the distance you are standing from the chart, which is 20 feet in the United States. The bottom number indicates the distance at which a person with “normal” vision can read the same line of letters you are reading.
A score of 20/20 means that you can see at 20 feet what a person with normal vision also sees at 20 feet. If your score is 20/40, it signifies that you must be 20 feet away to read a line that someone with normal vision can read from 40 feet away. Conversely, a score of 20/15 suggests better-than-average visual acuity. Legal blindness is often defined as having visual acuity of 20/200 or worse in the better-seeing eye, even with correction.
What Can Influence Test Accuracy
Several factors can impact the outcome of a visual acuity test. The results represent a snapshot of your vision under specific conditions. For instance, the level of illumination in the room is important; a poorly lit room can make it more difficult to discern the letters on the chart.
Your own physical and mental state also plays a part. Fatigue or a lack of concentration can lead to a performance that doesn’t reflect your true visual capabilities. Temporary issues, such as having dry eyes, can also blur vision and influence the test results. This is why the test is considered just one piece of a comprehensive eye health examination.