Assessing vision primarily refers to evaluating visual acuity, which is the sharpness and clarity of sight. This measurement determines how well someone can distinguish fine details and shapes at a specific distance. It provides a standardized way to quantify a person’s ability to see clearly.
Measuring Distance Vision with the Snellen Chart
The most common method for evaluating distance visual acuity involves the Snellen eye chart. This chart features rows of letters that decrease in size from top to bottom. During an eye examination, individuals typically stand or sit 20 feet (about 6 meters) away from the chart. To assess each eye independently, one eye is covered while the other reads the smallest line of letters possible.
The test begins with the largest letters at the top, and the patient reads down the chart until they can no longer clearly identify them. This process measures the sharpness and clarity of central vision. The familiar “20/20 vision” represents a benchmark for normal visual acuity, indicating that a person can see clearly at 20 feet what should normally be seen at that distance.
Decoding Your Visual Acuity Score
Visual acuity test results are typically expressed as a fraction, such as 20/20 or 20/40. The top number, or numerator, consistently represents the standardized distance between the person and the eye chart, which is 20 feet in the United States. The bottom number, or denominator, indicates the distance at which a person with normal vision could read the same line that the individual being tested read from 20 feet away.
For example, a score of 20/40 signifies that a person can see at 20 feet what someone with normal vision can see from 40 feet away. This means their vision is less sharp than average, as they need to be closer to discern details that a person with 20/20 vision could see from further away. Conversely, a score like 20/15 indicates better-than-average visual acuity, meaning the individual can see at 20 feet what most people can only see at 15 feet. A lower denominator suggests sharper vision. Visual acuity of 20/200 or less is often classified as legal blindness.
Assessing Near Vision and Other Optical Factors
Beyond distance vision, comprehensive eye exams also assess near vision, which is crucial for reading and close-up tasks. Near vision is commonly evaluated using a Jaeger chart or similar reading cards, typically held about 14 inches (36 centimeters) from the face. These charts feature paragraphs of text in varying font sizes, allowing the eye care professional to determine the smallest print an individual can read comfortably at a typical reading distance. This measurement helps identify conditions like presbyopia, an age-related loss of near focusing ability.
Other optical factors are also quantified during an eye exam. Astigmatism, a common refractive error where the eye’s front surface has an irregular curve, is measured using “cylinder” and “axis” values. The cylinder measurement indicates the degree of astigmatism, while the axis specifies the orientation of this curvature in degrees. These precise measurements allow for the accurate correction of blurry or distorted vision caused by astigmatism. For individuals with presbyopia, an “add power” is determined, which specifies the additional magnifying power needed in corrective lenses for clear near vision and is usually positive to enhance focus for reading and other close work.