Visual acuity measurements assess how clearly you see at a specific distance, typically using a Snellen chart. Here’s how to read and interpret them:

1. Format: Visual acuity is expressed as a fraction (e.g., 20/20, 20/40). The first number represents the testing distance (usually 20 feet in the U.S.). The second number indicates the distance at which a person with normal vision can read the same line you can read at 20 feet.
2. Interpretation:
   • 20/20: Normal vision. You can see at 20 feet what a person with normal vision sees at 20 feet.
   • 20/40: You see at 20 feet what a person with normal vision sees at 40 feet. This indicates milder vision impairment (e.g., you need to be closer to see clearly).
   • 20/200 or worse: Considered legal blindness in many regions if uncorrectable with glasses or contacts.
3. Additional Notes:
   • OD/OS/OU: Results may be listed for the right eye (OD), left eye (OS), or both eyes (OU).
   • With/Without Correction: Measurements may specify if taken with glasses/contact lenses (“corrected”) or without (“uncorrected”).
   • Other Systems: Some countries use metric (e.g., 6/6 instead of 20/20, where 6 meters is the testing distance) or decimal scales (e.g., 1.0 = 20/20, 0.5 = 20/40).
4. Context:
   • Testing is typically done one eye at a time, covering the other eye.
   • If you can’t read the largest letters, other methods (e.g., counting fingers, detecting hand motion, or light perception) may be used.