The history of the Snellen eye chart
Invented in 1862 by Dutch ophthalmologist Herman Snellen, the Snellen eye test chart remains the global standard for measuring distance visual acuity. The chart consists of eleven lines of block letters (optotypes) that decrease in size as you read downward.
How the 5x5 grid works
Snellen optotypes are not standard typographic fonts. Each letter is meticulously drafted on a 5x5 grid where each stroke, gap, and limb subtends exactly 1 minute of arc (1/60th of a degree) on the retina when viewed from the designated test distance. A 20/20 letter at 20 feet subtends an overall visual angle of 5 minutes of arc.
Understanding the 20/xx fraction
The numbers in Snellen notation describe a ratio comparing your vision to a standardized reference eye:
- Top number (20): The distance in feet from which you are viewing the chart.
- Bottom number (e.g., 20, 40, 100): The distance from which a person with standard normal visual acuity could read that same line of letters.
If your score is 20/40, you must stand at 20 feet to read what a person with normal vision could comfortably read from 40 feet away. If your score is 20/20, your distance visual acuity matches the accepted clinical baseline.
Why 'Line 5' is famous at the DMV
In driver licensing facilities and online forums, people often ask about "line 5 dmv eye test". On standard Snellen charts calibrated for 20-foot viewing distances, Line 5 is typically the 20/40 line (usually containing letters like P-E-C-F-D). Because 20/40 is the statutory minimum requirement to obtain an unrestricted driver's license in most US states, Line 5 is the crucial cutoff that examiners test.
Why static wall posters mislead
Many people print an eye chart poster or pull up an image on a smartphone. However, an uncalibrated digital image has an arbitrary pixel size that changes with screen resolution and magnification. If the letters are rendered 20% too large or too small, a true 20/40 letter can be easily misjudged as 20/20 or 20/60. To get an accurate measurement, use our card-calibrated online vision test, which scales vector optotypes to exact physical millimeter standards.