The Snellen eye chart is the familiar chart of letters used to measure distance visual acuity. It helps determine how clearly a person can see at a standard distance, usually 6 metres or 20 feet.
How it is used
Each line corresponds to a different level of visual detail.
- The patient covers one eye and reads the smallest line they can see
- Results are recorded as a fraction, such as 6/6 or 20/20
- Testing is repeated with the other eye and then with glasses if worn
- Helps decide whether glasses are needed or if vision has changed
Limitations
The Snellen chart measures sharpness, not overall eye health.
- Normal acuity does not rule out early glaucoma, macular disease, or field loss
- Crowded letters and lighting can affect performance
- Often combined with other tests in a full eye examination
- Children or non‑readers may need picture or matching charts instead
