The question of whether 15/20 vision is superior to 20/20 vision is common and easily answered by understanding how visual clarity is measured. The simple answer is yes, 15/20 vision represents a sharper level of distance sight than 20/20 vision. While 20/20 is the widely accepted benchmark for “normal” vision, it is not the highest level of visual acuity achievable. The measurement is derived from the standardized Snellen chart, a tool used globally by eye care professionals to assess a person’s ability to discern fine details at a distance.
Decoding the Snellen Vision Fraction
Visual acuity is recorded as a fraction, such as 20/20, which is a method of comparing a person’s eyesight to a standard of what is considered normal vision. The first number, or the numerator, represents the distance in feet at which the patient is standing from the eye chart during the test, which is almost always 20 feet in the United States. This distance is standardized because it approximates “optical infinity,” meaning the eye’s focusing muscles are relaxed as if looking at a very distant object.
The second number, the denominator, signifies the distance in feet at which a person with normal vision would be able to read the same line of letters the patient successfully read at 20 feet. For instance, if a person has 20/40 vision, they must stand at 20 feet to clearly see a line of letters that a person with normal sight can see from 40 feet away. The size of the letters on the chart is geometrically consistent, so a smaller denominator indicates better visual performance.
The Snellen fraction is essentially a ratio of the patient’s viewing distance to the distance at which a reference eye can see the same target. A result like 20/15 or 20/10 has a denominator smaller than the numerator, indicating that the person can see details at 20 feet that a reference person must move closer to see. Conversely, a larger denominator, such as 20/200, means the patient can only see at 20 feet what the reference person can see from 200 feet.
Defining Normal and Superior Visual Acuity
The 20/20 measurement is defined as the standard for normal, unaided distance vision, meaning that a person can clearly see at 20 feet what the average person can also see clearly at 20 feet. This standard is the target for most vision correction procedures and prescriptions. Approximately 35% of adults have 20/20 vision without any corrective lenses, though a much larger percentage achieve it with glasses or contacts.
A visual acuity of 15/20 is mathematically and functionally superior to 20/20 because the fraction is a ratio. While it is often expressed as 20/15 for consistency in the testing distance, the meaning is that the person can see at 20 feet what a person with 20/20 vision would need to move to 15 feet to see clearly. This means the individual with 15/20 vision can resolve details that are 25% smaller than the details a 20/20 person can resolve at the same distance.
This level of performance, sometimes referred to as “supra-normal” vision, is often seen in individuals with young, healthy eyes. To visualize the difference, consider a traffic sign: a person with 20/15 vision can read that sign from 20 feet away, while a person with 20/20 vision would have to walk five feet closer to read the same letters with the same clarity.
What Snellen Acuity Scores Do Not Measure
The Snellen visual acuity score is a measure of visual sharpness or clarity at a distance, but it provides an incomplete picture of overall visual function and eye health. This single metric does not account for several other abilities that contribute to how a person sees the world. The score is determined under high-contrast conditions, typically black letters on a bright white background, which is a simplification of real-world viewing conditions.
The test does not measure contrast sensitivity, which is the ability to distinguish an object from its background—such as a gray squirrel on a gray road on a rainy day—a significant factor in night driving and low-light environments. Furthermore, Snellen acuity does not evaluate peripheral vision, which is the side vision outside the direct line of sight.
Other important visual skills not assessed by the Snellen chart include depth perception, which is the ability to judge the relative distance of objects, and color vision. The test also does not provide information about the health of the eye itself. A person could have 20/20 vision but still have underlying eye diseases like glaucoma or early cataracts, which affect the visual system without immediately impairing central distance sharpness.