Visual acuity measures the sharpness or clarity of a person’s vision, assessing the eye’s ability to distinguish fine details and shapes. In the United States, this clarity is commonly expressed as a fractional score, known as the Snellen fraction. This fraction is derived from a standardized eye chart examination. Understanding the relationship between the two numbers in this fraction is the key to interpreting a vision score.
Decoding the Visual Acuity Fraction
The Snellen fraction, such as 20/40 or 20/20, represents a comparison of distances measured in feet. The numerator, which is always 20 in the standard test, signifies the 20 feet distance at which the person stands from the eye chart.
The denominator, the second number in the fraction, indicates the distance at which a person with normal vision can clearly read the same line of letters. A score of 20/20 is the baseline for normal visual acuity, meaning the individual can see at 20 feet what the average person can also see at 20 feet. A smaller denominator indicates superior vision, while a larger denominator suggests reduced acuity.
The Meaning of Superior Vision (20/10)
Yes, 20/10 vision is better than 20/20, as the smaller denominator signifies greater visual performance. A person with 20/10 vision can discern fine details on the eye chart from 20 feet away. Someone with normal 20/20 vision would only be able to accomplish this feat by moving 10 feet closer to the chart.
The individual with 20/10 acuity possesses vision that is twice as sharp as the standard. This level of visual precision is considered superior and is quite rare in the general population. While 20/20 is often described as “perfect” vision, 20/10 demonstrates an above-average ability to resolve visual details at a distance.
Context for Other Vision Scores
Scores with a denominator larger than 20 indicate reduced visual acuity. For instance, a score of 20/40 means the person must be 20 feet away to see what a person with normal vision can see from 40 feet away. This shows the individual needs to be closer to an object to see it with the same clarity as the average person.
As the denominator increases, the visual impairment becomes more pronounced. A score of 20/200 or worse in the better-seeing eye, even with the best possible correction from lenses, is the commonly accepted threshold for legal blindness in the United States. This means the person can only see at 20 feet what a normal person can see from 200 feet away.