Most people have experienced the routine eye examination, where an eye care professional asks them to read letters from a chart. This process determines the clarity of vision at a distance, known as far vision or distance visual acuity. Far vision measures the eye’s ability to resolve fine detail, based on the optical demands placed on the eye’s focusing system.
The Standard Clinical Measurement Distance
The universally accepted distance for testing far vision in clinical settings is 20 feet, or 6 meters. This measurement is the foundation of the visual acuity fraction used by eye care professionals. At 20 feet, light rays from the chart are virtually parallel when they reach the eye, simulating the conditions of looking at an object infinitely far away. This standardized distance ensures the eye’s internal focusing lens is in a relaxed state, allowing the clinician to accurately measure the eye’s refractive error without interference from focusing effort.
Understanding Optical Infinity
The concept of far vision is directly linked to optical infinity, which is the distance where light rays entering the eye are essentially parallel. This requires minimal focusing effort, or accommodation, from the lens. For the human eye, this point is approximated at 20 feet, serving as the functional equivalent of infinity for a routine eye exam.
The Meaning of Visual Acuity (20/20)
Far vision clarity is quantified using the Snellen fraction, commonly known as the 20/20 measurement. The numerator (20) represents the standard testing distance in feet. The denominator indicates the distance at which a person with normal vision could read the same line of letters. For example, 20/20 vision means the patient reads at 20 feet what a person with normal vision reads at 20 feet, establishing the benchmark for clear distance vision. A result of 20/40 means the patient must be 20 feet away to see letters that a person with normal vision can see from 40 feet away. Conversely, 20/15 signifies better-than-average distance vision.
Delineating Far Vision from Near and Intermediate Vision
Far vision represents the ability to see clearly from the standard measurement distance outward to optical infinity, traditionally beginning at 10 to 20 feet. Tasks relying on far vision include driving or recognizing faces from a distance. Near vision covers the reading distance, generally defined as 14 to 16 inches from the face, used for tasks like reading a book or viewing a smartphone. Intermediate vision falls between near and far, typically spanning from about 2 feet to 10 feet, and includes activities such as working on a computer monitor or viewing items on a supermarket shelf.