The vision scale is a standardized way of expressing how sharp your eyesight is, most commonly written as a fraction like 20/20 or 20/40. The first number is the distance you stand from the eye chart (20 feet in the U.S.), and the second number is the distance at which someone with perfect eyesight could read that same line. So if you have 20/40 vision, you need to be 20 feet away to read what a person with normal sight can read from 40 feet away.
How the Snellen Scale Works
The most familiar vision scale is the Snellen chart, the poster of black letters that shrink line by line. During a standard eye exam, you stand 20 feet from the chart and read the smallest line you can. The bottom number of your result reflects which line that was. A result of 20/20 is considered the baseline for normal vision, while 20/200 means your eyesight is significantly reduced.
Countries that use the metric system measure from 6 meters instead of 20 feet, so “normal” vision is written as 6/6 rather than 20/20. The math is the same. Here are some common conversions:
- 20/20 = 6/6
- 20/40 = 6/12
- 20/100 = 6/30
- 20/200 = 6/60
20/20 Isn’t Actually “Perfect” Vision
Many people assume 20/20 is the best eyesight possible. It’s not. It’s simply the baseline that Hermann Snellen chose in the 1860s, and it may have been set too conservatively. Some researchers have argued that if Snellen had converted his original measurements properly, the normal standard would have been 20/10.
Human eyes can do much better than 20/20. A 1996 study of Los Angeles Dodgers players found that 77% had vision of 20/15 or better, 42% measured 20/12.5 or better, and the sharpest recorded acuity was 20/8.9, roughly 2.25 times better than the standard. The theoretical limit of the human eye depends on the density of light-detecting cells in the center of the retina, which varies widely from person to person and corresponds to a range from about 20/10.5 to 20/7.1. Even some patients in their 80s can achieve 20/10 after cataract surgery.
What the Numbers Mean in Everyday Life
Vision scale results translate directly into practical thresholds. For driving, the U.S. Federal Motor Carrier Safety Administration requires at least 20/40 in each eye, with or without glasses, for commercial vehicle operators. Most state DMVs set a similar cutoff for a standard license. If your acuity falls below that, you may be restricted to daytime driving or required to wear corrective lenses.
Legal blindness in the United States is defined as best-corrected visual acuity of 20/200 or worse in your better eye, or a visual field narrowed to 20 degrees or less. Under current Social Security Administration rules, if you cannot read any letters on the 20/100 line of a modern chart, you qualify as legally blind. If you can read at least one letter on that line, you do not.
International Categories of Vision Loss
The World Health Organization classifies vision impairment into graded categories based on the Snellen scale:
- Mild impairment: worse than 20/40 but 20/60 or better
- Moderate impairment: worse than 20/60 but 20/200 or better
- Blindness: worse than 20/200 but 20/400 or better
- Severe blindness: worse than 20/400 but still able to count fingers at 1 meter
- Very severe blindness: worse than finger counting, but some light perception remains
- Total blindness: no light perception at all
These categories determine eligibility for rehabilitation services, disability benefits, and assistive technology programs in many countries.
The LogMAR Scale
Researchers and clinical trials often use a different vision scale called LogMAR, which stands for the logarithm of the minimum angle of resolution. The Snellen chart has a practical problem: the number of letters per line varies, and the size jump between lines isn’t consistent, making it hard to track small changes in vision over time. The LogMAR chart (often called an ETDRS chart) fixes this by presenting five letters on every line, spaced at equal intervals. Each letter you read is worth 0.02 LogMAR units, so clinicians can score your vision with much finer precision.
On the LogMAR scale, 0.0 equals 20/20 vision, positive numbers indicate worse vision, and negative numbers indicate better-than-normal sight. A LogMAR of 1.0 equals 20/200. This scale matters most if you’re involved in a clinical trial or tracking a progressive eye condition, since it detects smaller shifts in acuity than the Snellen chart can.
Near Vision and the Jaeger Scale
The Snellen chart measures distance vision. For reading and close-up tasks, eye doctors use the Jaeger (J) scale. You hold a card about 14 inches from your face and read progressively smaller paragraphs of text. The results are labeled J1 through J16, with J1 being the smallest print (equivalent to about 20/20) and higher numbers indicating worse near vision. Some approximate equivalents:
- J1 = 20/20
- J2 = 20/30
- J4 = 20/50
- J8 = 20/100
- J16 = 20/200
Near vision testing becomes especially relevant after age 40, when the lens inside the eye gradually loses flexibility and close-up focus declines.
How Children Are Tested
Young children who don’t know their letters can’t use a standard Snellen chart, so eye doctors use symbol-based charts like Lea Symbols (a house, circle, square, and apple) or Patti Pics. These charts are calibrated to produce results on the same 20/20 scale. Studies comparing Lea Symbols to standard adult letter charts in adults found no clinically significant difference in the acuity scores, confirming that symbol charts are a reliable stand-in. There can be small variations between different symbol sets, typically about half a line on the chart, but not enough to change clinical decisions.
Why Acuity Isn’t the Whole Picture
The standard vision scale measures one specific thing: your ability to resolve fine detail at high contrast (black letters on a white background). But real life rarely looks like an eye chart. Contrast sensitivity, a separate measurement, captures how well you can distinguish objects from their background when the difference between them is subtle, like spotting a gray curb against gray pavement or driving in fog.
Two people with identical Snellen scores can have very different contrast sensitivity depending on the condition affecting their eyes. In people with cataracts, contrast sensitivity drops only slightly as acuity worsens, because cataracts blur everything relatively evenly. In people with retinitis pigmentosa, contrast sensitivity drops almost in lockstep with acuity, nearly one-for-one. Conditions like macular degeneration and glaucoma fall somewhere in between. This is why a full eye exam typically includes more than just reading the letter chart.