What is the proper testing distance?

Since visual acuity is defined by the angle under which letters are viewed, its measurement can be done at any distance, provided that the scale is adjusted for the distance used. When a letter chart is used as a target for subjective refraction, the viewing distance is important, because the longer the viewing distance, the more accommodation will be relaxed.

Snellen’s original charts were calibrated for a viewing distance of 20 Parisian feet. As soon as the Treaty of the Meter was signed (1875) he converted to metric distances and made charts for 5 meters (more convenient with the decimal system) and 6 meters (closer to 20 feet). These distances have the advantage that they relax accommodation and that small forward movement of the patient have a negligible influence.

The NEI ETDRS protocol reduced the viewing distance to 4 meters to have a more manageable chart size.

For testing of children, a 3 meter distance (10 ft) is often used since it is easier to keep their attention at that distance. For smaller areas or shorter examination lanes, charts for 2.5 meters and 2 meters are available. For testing of Low Vision patients, a 1-meter distance is advantageous, since it can cover a much wider range of visual acuity values. A typical 1-meter chart covers visual acuities from 20/1000 to 20/20. It requires 1 D over the distance correction.

For testing of reading acuity, various distances have been used. A distance of 40 cm is commonly recommended as standard near vision.

Most charts for use at 1 meter or less have a cord attached to facilitate maintaining the accurate viewing distance.