Visual acuity measures the clarity and sharpness of distance vision, usually assessed during a routine eye exam. The Snellen eye chart, featuring rows of letters that decrease in size, is the most common tool used for this test. The result is expressed as a fraction that compares your vision to the established standard for normal sight.
Decoding the Snellen Fraction
The Snellen fraction quantifies visual acuity using a specific mathematical relationship. The numerator, typically 20, represents the standardized testing distance in feet from the eye chart.
The denominator (100 in 20/100) represents the distance in feet at which a person with normal, 20/20 vision could clearly read that same line of letters. This means a person with 20/100 vision must stand 20 feet away to see what a person with normal vision can see from 100 feet. The larger the denominator, the poorer the distance vision.
The fraction measures only the eye’s ability to resolve fine detail at a distance. It does not account for other aspects of overall visual ability, such as color perception, side vision, or depth perception.
Visualizing 20/100 Vision
A person with 20/100 vision experiences significant blurriness when viewing distant objects. To visualize this, imagine standing 100 feet from a street sign that a person with 20/20 vision could read clearly.
For someone with 20/100 acuity, the sign would appear indistinct until they walked closer, down to 20 feet, before the letters became clear enough to read. The distant world is noticeably softer and less defined than it is for someone with normal sight. Identifying faces across a large room, such as an auditorium, would be challenging because the features would be too blurred to recognize.
This level of visual impairment is considered moderate, but it is far from the threshold for legal blindness in the United States. Legal blindness is defined as having a corrected visual acuity of 20/200 or worse in the better-seeing eye. While 20/100 vision substantially impacts daily activities like driving and recognizing distant objects, it is not classified as severe visual loss.
Causes and Correction Methods
The most frequent causes of 20/100 vision are refractive errors, which are imperfections in how the eye focuses light. Myopia (nearsightedness) is a primary culprit, occurring when the eyeball is too long or the cornea is too curved, causing light to focus in front of the retina. Astigmatism, involving an irregularly shaped cornea or lens, can also contribute to blurriness by causing light to scatter and focus at multiple points.
Fortunately, 20/100 vision caused by refractive error is highly correctable. A comprehensive eye examination allows an eye care professional to precisely measure the error and prescribe corrective lenses. These lenses can be eyeglasses or contact lenses.
These devices introduce the optical power needed to redirect light precisely onto the retina, immediately improving visual acuity. For many, prescription lenses can restore vision to 20/20 or better. Refractive surgery procedures, like LASIK, can also be performed to reshape the cornea for a long-term correction.