The standard scale used to describe one's visual acuity, which is typically assessed with a Snellen acuity chart. 20/20 represents baseline (normal acuity). Practically, this means that the patient can read letters on a high-contrast eyechart 20 feet away that a person with normal acuity can also read from 20 feet away. A denominator larger than 20 (e.g. 20/50) represents sub-normal acuity, where the patient can read from 20 feet what a person with normal acuity can read from 50 feet away. A denominator smaller than 20 (e.g. 20/10) represents superior acuity, where the patient can read from 20 feet what a patient of normal acuity can only read from 10 feet away.

20/20 is often referred to as "normal vision" or even "perfect vision". This is inaccurate and can be misleading (even dangerously so, for the prospective laser surgery patient).

A "normal" score of 20/20 does not mean normal vision; it only means normal acuity, which measures one part of the visual system. One can have normal acuity and at the same time have vision quality defects such as double vision, poor contrast, night vision disturbances (starbursts, haloes, etc). The prospective patient needs to specifically aim to achieve both good acuity and good vision quality. The complications patient who is puzzled by being told he has "excellent vision" when he does not see well needs to ask specifically for testing aimed at assessing vision quality, such as contrast sensitivity testing and wavefront aberrometry.

In most places 20/40 is also "driving standard vision", that is, one who can read the 20/40 line on the eyechart meets the visual requirements to be given a driver's license. The prospective laser surgery patient, however, needs to recognise that this does not by any means guarantee that the patient will see well enough to feel safe driving, particularly at night.

In laser vision correction, an uncorrected visual acuity (that is, the patient's acuity without using glasses or contacts) of 20/20 is the typical ideal goal of surgery. 20/40 represents the standard of success. Acuity is the only uniformly adopted standard of success in laser vision correction today.

visual acuity

20/happy

driving standard vision

Snellen eyechart

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Last updated 1 Dec 2004.