The Eyes & Vision

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 fingers 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. 

Illustration above is from  webvision.med.utah.edu showing how an eye with 20/20 vision has the acuity to identify a letter that subtends an angle of 5 minutes.  Each bar of the E as well as the spaces between the bars subtends and angle of 1 minute of arc.

The basic principle of the modern Snellen Chart: If a patient is placed at a distance of 20 feet (6 meters), he or she should be able to recognize the very same letter if it's twice the size when viewed at 40 feet. 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 the 20 foot test letter; and  he is able to recognize the a letter of 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.  

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.                                                              

This diagram from Peter K. Kaiser's The Joy of Vision shows the eye, often called the globe, within the bony orbit of the skull, showing three of the six extraocular muscles which control the direction the eye is pointing, and showing the eyelids, optic nerve and the two chambers of the eye: the anterior chamber containing the aqueous humor, and the posterior chamber containing the vitreous humor, and the retinal blood vessels which line the posterior chamber and enter the eye with the optic nerve.

The diagram of the eye above shows the clear cornea at the front of the eye, and its relation with the iris. The Angle produced where the posterior (back) of the cornea intersects the anterior (front) of the iris is significant. If the angle is too narrow it can block the normal flow of aqueous fluid produced in the anterior chamber from escaping into the Canal of Schlemm, causing the pressure to increase in the eye and producing narrow angle or angle closure glaucoma.  Covering the sclera at the front of the eye is a clear  tissue called the bulbar conjunctiva (labelled conjunctiva in the illustration).  The conjuctiva also covers the inside portion of the lids, where the lids make contact with the eye and where it is known as the palpebral conjunctiva. The cornea, crystalline lens inside the eye, and the vitreous normally have no blood vessels, but the conjunctiva is highly vascularized, and becomes easily inflammed (a condition known has conjunctivitis). The most posterior and most central part of the retina is the macula, and the center of the macula is the fovea. The greatest concentration of the photoreceptive cells, especially cones, is in this area, and the high density of these cells in this area is necessary to produce sharp visual acuity. 

The picture on the left from biology.clc.uc.edu shows a 400x magnified Cross section of the retina. The photoreceptors (rods and cones) are seen as the back layer of the retina, behind Muller's fibers, Ganglion cells, Amacrine cells, bipolar cells, Horizontal cells and the Outer limiting membrane. Behind the photoreceptors are pigment cells and the two layers of the eye behine the retina--the vascularized choroid and the connective tissue outer layer, the Sclera. Not shown are the retinal blood vessels in front of the Muller's fibers.  Although light must travel through retinal blood vessels and all the cells in front of the photoreceptors the brain filters out the cells and vessels in front of the photoreceptors.

The interactive diagram presented on Peter Kaiser's Joy of Visual Perception is a wonderful aid in illustrating the parts of the eye and how they function. Click the underlined title above to go to Kaiser's ingenious creation.  It is best to refer to the pictures when reading descriptions of eye conditions and diseases, as well as the workings of the normal eye. 

In the normal eye, light from a distant object passes first through the clear front curved window of the eye, the cornea, where the light rays are bent or refracted, then through the anterior chamber, which is filled with a watery substance called the aqueous humor, then through the crystalline lens of the eye, where the light is refracted again.  Light then passes through the clear gelatinous vitreous body, before focusing on the photoreceptive cells lining the retina inside the back of the eye.
 
The image that is focused on the retina is inverted. The brain rights the inverted image and filters out several things that are in front of the retina (such as retinal blood vessels) which are not perceived in the final picture. The sharpness of the image focused on the retina is influenced by many things, including the clarity of the media (which diminshes as we age with such conditions as cataracts and vitreous floaters), refractive errors (myopia, hyperopia and astigmatism), and other media produced aberrations, and the density of photoreceptive cells in the retina. [Note: The refractive errors that are caused by these aberrations that are not measured in standard myopia, hyperopia and astigmatism testing can be analyzed using computer wavefront technology and programmed into certain eximer laser instruments in refractive surgery. Except for its limited role for this purpose wavefront technology has little relevance to the treatment plan for people with refractive errors corrected in typical manner--it currently has no application in lenses used in eyeglasses, contact lenses, or intraocular lens implants, but does influence everyone's visual acuity to some (usually insignificant) degree.]

Age	Diopters	Inches
10	14.00	2.81
15	12.00	3.28
20	10.00	4.94
25	8.50	4.63
30	7.00	5.63
35	5.50	7.16
40	4.50	8.75
45	3.50	11.25
50	2.50	15.75
55	1.50	26.25
60	.75	52.49
65	.25	157.48
70	0	0

The diagram from Peter Kaiser's Joy of Visual Percetion shows elements of the eye needed for focusing on near objects.  The structure of the normal healthy young eye allows the cystalline lens inside the eye to focus on near objects when a nerve impulse from the brain via the third cranial nerve causes the ciliary muscle, behind the iris, to contract, this causes the fibers (called zonules) attached to the lens capsule surrounding the crystalline lens to loosen, causing the lens to become more convex (bulging more in the center), which shortens the focal length of the lens. Dr Wallace's LA Sight website has a wonderful Flash animation showing how the crystalline lens becomes convex when focusing on a near object.   The change of focus of the lens to allow clear focus of near objects is called accomodation and can be measured in diopters (Dioptor = 1/focal length of lens in meters).

The ability of the lens to accomodate, to change shape in order to focus on near objects, is dependent on the flexibility of the lens.  The lens becomes harder and less flexible with age.  In the 19th century Dutch physiologist Franciscus Donders (1818-1889) knew  this, and created the famous Donders Table of Accomodation (see Table to left) to illustrate the point. From the table we can see that a teenager has the ability to accomodate about 12 diopters, while a 40 year old can only accomodate about 4.50 dioptors. This decrease of accomodation with age is a universal phenomenon.  To a small extent, the decrease of near focus is offset by the fact that the pupil size tends to decrease with age and it is a fact of physical optics that aperture diameter and depth of focus are inversely proportional--that is, a smaller pupil has an increased depth of focus, and therefore a near object is clearer with the smaller pupil than with a larger pupil. This is the principle on which the camera obscura (pinhole camera) and other pinhole lens devices are based.   And it is why one hears  about someone in his eighties who can see at all distances with each eye without the aid of any corrective lens. If you look at the pupil size of that 80 year old,  it will look more like a pinhole.   The fact is that  by the time one reaches the age of 45, the amount of lost accomodation generally has made the ability to focus on close objects very difficult, especially in dim light (when the pupil has naturally dilated to a larger size).  This lack of accomodation due to age is called presbyopia (See Diseases and Conditons).