What does "visual acuity" of 20/40 indicate?

The concept of "visual acuity" is a measure of the clarity or sharpness of vision as determined by the ability to discern letters or symbols at a standardized distance. A visual acuity of 20/40 indicates that the patient can see at 20 feet what a person with normal vision can see at 40 feet. This means that the patient's vision is below the standard level (which is considered 20/20), indicating a degree of impairment.

In practical terms, this measurement implies that objects at 40 feet are perceived as clearly by a person with 20/40 vision as they would be by someone with normal vision standing only 20 feet away. Understanding visual acuity in this way helps clinicians assess and classify the severity of vision deficiencies, providing essential information for diagnosis and treatment.