Visual acuity is a simple and useful measure of the ability of the eye to resolve fine detail. It is an essential component of the assessment of all patients presenting to the ED with an eye problem. Traditionally, it has been tested by asking the patient to read letters (known as optotypes) from a chart, commonly the Snellen chart.

The chart is placed at a set distance, usually 6m, and each eye is tested individually by covering the other eye.

The letters are sized according to distance, i.e. the largest letter, size 60, can be seen at 60m by a person with normal vision.

The acuity is recorded as a fraction, with the numerator as the distance from the patient to the chart, over the denominator as the lowest line that more than half the letters can be seen, e.g. 6 / 12 for a patient at 6m who is able to see the letters on the size 12 row. If the patient is unable to see some of the letters on a line, the acuity is recorded as: