Visual acuity is a standard measure of how clearly a person sees, representing the sharpness of vision. Eye care professionals use standardized metrics during eye examinations to determine the performance of the visual system. This measurement determines the smallest detail a person can distinguish from a specific distance. Scores are expressed as a fraction to compare an individual’s vision to the established average.
Interpreting the Visual Acuity Fraction
Vision scores are commonly expressed as a fraction, such as 20/20 or 20/15. The numerator indicates the testing distance, typically fixed at 20 feet in the United States. The denominator represents the distance at which a person with standard vision can clearly read that line.
Standard visual acuity is 20/20. 20/15 vision means an individual can see an object clearly at 20 feet that a person with 20/20 vision would need to move to 15 feet away to see clearly. This indicates that 20/15 vision is better than average, resulting in sharper distance vision.
How the Snellen Test Measures Vision
Visual acuity measurement relies on the Snellen eye chart, developed in the 1860s by Herman Snellen. This chart uses rows of capital letters, known as optotypes, that decrease in size down the chart. The eye care professional asks the patient to read the smallest line of letters they can see clearly from 20 feet away.
In clinical settings lacking a 20-foot testing lane, mirrors or calibrated digital screens simulate the required distance optically. The last line a patient reads accurately determines the denominator of their visual acuity score; for instance, reading the line marked with the 15-foot designator results in a 20/15 score.
Biological Factors Influencing Acuity
Achieving high clarity like 20/15 depends on several anatomical and physiological factors within the eye. The eye’s ability to precisely focus light onto the retina relies heavily on the healthy curvature of the cornea and the flexibility of the lens. Irregularities in these structures can introduce a refractive error, blurring the image.
Sharpest central vision is achieved at the fovea, a small depression in the center of the retina containing a dense concentration of cone photoreceptor cells. This area is specialized for high-resolution vision, allowing light to strike the photoreceptors directly. Visual acuity reflects how effectively the eye’s optics focus the image onto this highly sensitive region.