---
title: Snellen eye chart
description: "The [Snellen eye chart](https://bluefinvision.com/snellen-test-chart-online/) 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…"
url: "https://bluefinvision.com/knowledge-base/topics/diagnostics-testing/snellen-eye-chart/"
updated: "2026-01-22"
category: "Diagnostics & Testing"
---

# Snellen eye chart

The [Snellen eye chart](https://bluefinvision.com/snellen-test-chart-online/) 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