The Essence of Vision--How is It Measured? What is 20/20?
by John W. Elman, OD
What is good vision? What is normal vision? How do we measure the differences in vision between people? It is important to develop a system, a numeric scale, to determine visual acuity and rate the vision of someone with less than normal vision to what is considered normal. For people with extremely poor vision, people who are totally or partially blind, a descriptive, non-numeric designation is still used by eye doctors. Totally blind eyes are described as NLP for "No Light Perception"; those who can only tell the difference between a light being present or not are said to have LP "Light Perception"; those that see movement but cannot identify objects are designated as HM for "Hand Motion"; and those that cannot identify anything on an eye chart, but can count the number of finger
s on the examiner's hand which is held within a couple feet of their eye have a visual acuity rating of CF for "Counting Fingers", usually accompanied by the distance this was done (as "CF @ 2 ft," "CF @ 3 ft," etc). People with vision better than CF need a more precise, universally accepted, numeric scale to measure visual function. The basic assessment of visual function has to do with visual acuity. It is generally the assessment of visual acuity that determines whether treatment (lenses, surgery, medications or other) should be done and if treatment is done, whether it is successful. These questions were pondered by a couple of prominent 19th century scientists, Hermann Ludwig Ferdinand von Helmholtz (1821-94) and Hermann Snellen (1834-1908). Helmholtz was a major influence on German science during the mid-nineteenth century. He was a physicist and psychologist who made major contributions to both fields. Helmholtz's Treatise on Physiological Optics is widely recognized as the greatest book ever written on vision. This classic work was translated into English to mark the centenary of Helmholtz's birth and is one of the most frequently cited books on the physiology and physics of vision.
Helmholtz's Treatise transformed the study of vision by integrating its physical, physiological and psychological dimensions. He provided the explanation of the mechanism of accommodation, invented the ophthalmoscope, revived the three-colour theory of vision first proposed in 1801 by Thomas Young, invented the telestereoscope, produced some novel visual illusions, and argued for the involvement of knowledge in perception. The work was originally published in German, as Handbuch der physiologischen Optik, in three separate volumes between 1856 and 1866, and then together in 1867.
What is the smallest thing a normal eye can see? What are the limits of visual acuity? On what factors does visual acuity depend?
Visual acuity depends not only on such physical properties as the health and refractive errors of the eye (myopia, hyperopia, astigmatism), but the physical health and integrity of the neural pathway from the retinal photoreceptors to the visual cortex of the brain, and also psychological factors. You are more likely to correctly identify something you are expecting to see than to identify something you weren't expecting. To find the limit on how good an eye might see we will use an eye with no refractive error, perfect health of the nervous system, healthy circulatory system, perfectly clear media both in the eye and in the environment of the test object in front of the eye. Let's assume that the object is well lit, bright, of high contrast to its background, then the smallest object seen by this ideal eye would only be limited by the density of photoreceptive cones in the fovea in the center of the retina of the eye.
Let us consider a couple of thresholds of vision and some examples. Let's assume you were waiting at a bus stop for a bus that had its destination printed above the windshield. You knew the bus was going to be approaching from a far distance on a long straight road. It is a clear day, and from your vantage point you can see at least 20 miles down the road. You peer as far down the road as you can and you see nothing approaching. Then you see that there is something coming--but you can't tell if it is a person on a bike, a car, a truck, or a bus. This initial sighting of the distance object would represent an absolute threshold of vision. It is the point where you can see something but before you can identify it. As it approaches you can see that it is indeed a bus. This is your threshold of visual recognition. As it gets even closer you can identify the lettering above the windshield on the bus. This is another threshold up the recognition scale. As the bus gets closer and closer it looks larger and larger to you, and it encompasses an increasingly larger amount of your visual field of view. You see more and more details, the front wheels of the bus, the windshield, and as it stops in front of you you can see the drivers face behind the windshield and as you look at the bus driver your entire visual field is filled with the bus.
The visual field is the whole area an eye can see. If you look straight ahead without moving your eyes, for each eye the normal monocular visual field is a slightly irregular oval which measures, from fixation (the point you are looking at, such as a center point of the driver's face), approximately 60 degrees upward and 60 degrees inward, 70 to 75 degrees downward, and 100 to 110 degrees outward. Helmholtz determined the smallest thing that a normal eye could recognize, the threshold of visual recognition, as being an object that subtends an angle of 1' (one minute) of arc on your retina, regardless of the distance the object is from the eye. It could be an elephant a thousand feet away, or an insect a few feet away, or the letters on a sign across the room. A minimal area of your retina would have to be stimulated, and the minimal amount of your visual field involved for you to recognize the object would create at least 1' of arc according to Helmholtz.
Those of you who haven't studied geometry might ask, "what is 1' of arc?" A circle has 360 degrees and each degree of the circle can further be divided. Each degree contains 60' (60 minutes) and each minute contains 60 seconds (60"), so an angle of 1' (1 minute) is the same as 1/60 degree of arc, and that is the minimal degree of arc for visual recognition in the normal eye.
Several people invented charts that took into account Helmholtz's 1' of arc as the threshold of visual recognition in their design. In 1862 Dr Herman Snelling invented what would become the most widely used chart to measure Visual Acuity. The Snellen Chart consists of alphabet letters designed to subtend 5-minute angles to the eye at a distance of 20 feet. Each letter is enclosed in a square of 5' of arc, with the width of its arms and interstices subtending a 1' angle (Fig. 1). The design conforms with the distribution of cone-shaped elements in the retina that are primarily responsible for acute vision.
Fig. 1. Construction of the Snellen test letter in which A-to-B's width subtends a 5' angle; the arm-widths of c-to-d and the interstices subtend 1' angles.
The Snellen Chart's first line of type is constructed so that its angle is formed at a distance of 200 feet; the second at 120 feet; the third at 80 feet; the fourth at 60 feet; the fifth at 40 feet; the sixth at 30 feet, the seventh at 20 feet, plus additional lines subtending the same angle at 15 and 10 feet. Consequently, if a patient is placed at a distance of 20 feet, he should be able to recognize 20 foot test letter; and he is able to recognize the a letter of twice the size when viewed at 40 feet (Fig 2).
Fig. 2.
Basic principle of the modern Snellen Chart. If a patient is placed at a distance of 20 feet, he or she should be able to recognize the very same letter if it's twice the size when viewed at 40 feet.
Generally the testing of visual acuity is done at 20 feet. At that distance the 20/20 line on the Snellen Chart is the only one which has a letter subtending a total angle of 5' (minutes) of arc and with interspaces between the bars of 1' of arc. The 20/40 line has letters twice as large (subtending angles twice as large) at 20 feet, and the 20/200 letter is 10 times the size of the 20/20 letter at 20 feet, subtending an angle 10 times as large (the letter subtends an arc of 50' with spaces of each subtending 10' arcs). Most modern charts are based on the Snellen principle and have added a 20/400 letter, which is twice the size of the 20/200 letter and 200 times the size of the 20/20 letters.
What does 20/20 mean?
Visual acuity by Snellen's method is recorded in the form of a fraction. The numerator represents the distance at which the test was made, while the denominator represents the distance for which the letter was designed. Thus, 20/20 vision represents normal sight because it indicates an eye can recognize a letter from 20 feet away that it should be able recognize at 20 feet. 20/40 indicates defective vision; it means the eye can read a letter from no farther than 20 feet away that it should be able to recognize at 40 feet. 20/200 means the person examined can see at 20 feet what the normal can identify at 200 feet. The greater the denominator, the worse the vision. Conversely, 20/15 denotes superior sight because it indicates the eye can recognize a test letter from 20 feet away that an average eye cannot recognize from farther than 15 feet. Sometimes a metric version of Snellen acuity is used. Instead of the English measurement of "feet" as in "20 feet," meters are used. In the metric system 20/20 would be 6/6. Usually Snellen acuity is used to measure vision in the far distance, and the 20 foot distance used in an examining room is optically similar in determining lens power in refraction to "optical infinity." Optical infinity is infinitely far, like looking at a star in the heavens, or trying to see the stage from the last row of the Hollywood Bowl. The same ophthalmic lens that works at 20 feet should also be good at optical infinity. During the course of an eye examination there are usually multiple visual acuity tests done and written for each patient. Visual Acuity (VA) for each eye without corrective lenses (glasses), VA with corrective lenses for each eye, and best correctable visual acuity (BCVA) is given for each eye and for the eyes together. A reduced size Snellen Chart to measure vision for a near reading of 14 inches is also used in eye examinations. The near chart uses a Snellen fraction, as well as a scale developed by Jaeger, which calls 20/20 J1 ,20/30 J2, 20/50 J3 20/40 and goes up to J16 (20/200). The near chart also rates vision based on the 5' angle of arc used in the normal Snellen chart.
What is Legally Blind?
The definition of "Legally Blind" can involve either a severe reduction in the Visual Field (usually a visual field of less than 10% in the best eye) or Best Correctible Visual Acuity (BCVA) of less than 20/200 in the best eye. The definition for blindness has nothing to do with uncorrected vision--only the acuity with the best corrective lenses in front of the eyes.
Other Measurements of Vision
Other devices that are used to measure visual function include, charts that measure Contrast Sensitivity, visual Field assessment to measure peripheral vision and blinds spots in the visual field (called scotomas), color vision tests, Amsler Grid which measures the presence and degree of distortions in the center of vision (indicating a problem in the central macular region of the retina), stereoscopic vision (how the eyes work together to fuse the images of each eye to produce one image with depth), and visual evoked potential (which measures the speed of the neural impules from the retinal receptors to the brain). Although any of these tests may be done, the Snellen Acuity is the main test to measure visual function and is universally recognized as the standard.
