Snellendesigned his optotypes on a 5×5 grid, on which the line thickness is one unit and the letter width and height are five units. This system is followed for most letter and number charts. For children’s tests (such as Allen picturesand other testsymbols), it is often abandoned and much thinner lines are used. Precision Vision offers stylized pictures, called Patti Pics, for which Snellen’s 5×5 principle is followed. They result in less change in the measured acuity when children graduate from picture charts to letter charts. Alternative symbols that can be used for illiterate adults or in countries that are not familiar with the Roman alphabet are the tumbling E test and the Landolt C or broken ring.
Letter charts are not only used to measure visual acuity, they are also used as targets for subjective refraction. This is the main reason distance acuity is measured more often than near acuity. At a long distance, accommodation is relaxed, so that the refraction can be more accurate. At a longer test distance, the effect of small changes in the subject’s position is less important and can be ignored.
As today’s exam lanes often are smaller than 20 feet (6 meters), charts are often designed for shorter distances. This is no problem for visual acuity measurement, as long as the actual test distance is accurately accounted for. For use in refraction, shorter distances are less desirable. At 4 m (the recommended distance forETDRS charts) the accommodative demand is 0.25 Diopters and cannot be ignored. In smaller rooms, the use of mirrors is recommended to increase the test distance.
Letter size
Snellen used a rather verbose definition to indicate the letter size: “the distance in meters (feet) at which the letter subtends 5 min of arc.” Louise Sloan simplified this by defining the M-unit as the size that subtends 5 min of arc at 1 meter. Thus, the verbose statement “this letter subtends 5 min of arc at ‘x’ meter” can be simplified to “this letter is ‘x’ M-units”.Here the number denoting the M unit size is the viewing distance in meters for 20/20 acuity.
Most Precision Vision eye charts carry the letter size designation in M-units. This makes it easy to calculate the visual acuity if the chart is used at any other distance than the one for which it was designed by inserting the new test distance (in meters) and the letter size (in M-units) directly into the above formula.
Interpretation
It is a persistent urban legend that 20/20 would represent normal, average or even perfect vision. This is not so. Snellen deliberately chose his reference standard (5 min of arc) as a size that is “easily recognized by normal eyes.” Thus, almost all normal eyes will equal or exceed the reference standard. If 20/20 equaled average acuity, half of the population would fall short of 20/20, since that is the definition of average.
Visual acuity values are understood best by the following simple rule. On a Snellen chart we determine the line that the person can just recognize. If that line is twice as large as the reference standard (20/20), we state that that person’s MAgnification Requirement (MAR) is 2x. If the MAgnification Requirement is 2x, the visual acuity is 1/2 (20/40). Similarly, if the MAgnification Requirement (MAR) is 5x, the visual acuity is 1/5 (20/100); if MAR = 10, visual acuity = 1/10 (20/200), and so on.
The MAR and the visual acuity scale are opposites. A high MAR value indicates low or poor visual acuity; a low MAR value indicates good acuity. Their relationship is also true in reverse. A patient with 20/60 (1/3) visual acuity needs 3x magnification to reach the reference standard. This can be achieved with large print that is 3x normal, with a 3x magnifier or with a 3x telescope. Since normal vision is better than 20/20, some extra magnification is desirable for comfortable and sustainable performance.
LogMAR Notation
The MAR value is best known for its logarithm: logMAR. As for MAR, a higher logMAR value indicates poorer vision. Although the logMAR notation is often presented as a visual acuity notation, it actually is a notation ofvision loss. Zero logMAR indicates standard vision; zero visual acuity indicates blindness.
The logMAR notation is most convenient when used with a chart where the letter sizes follow a logarithmic progression, as on the ETDRS charts. Because of this, such charts are often referred to as logMAR charts. On these charts eachincreaseof 0.1 units on the logMAR scale indicates a one linelosson the visual acuity chart.
The logMAR scale is widely used in scientific publications where visual acuity values need to be depicted graphically to analyze trends or need to be averaged across population groups.
Visual Ability
Persons with 20/40 (0.5) visual acuity need 2x magnification. This does not mean, however, that they have lost 50% of their visual abilities. Clinicians have long used the expression “lines gained” or lines lost” to indicate changes in visual ability. The Visual Acuity Score (VAS) provides a convenient scale to estimate visual abilities. On this scale 20/20 is rated as 100; on charts with a logarithmic progression each line is worth 5 points and each letter read is 1 point. 50 VAS points stand for 20/200; it is reasonable to estimate that at this level the average person has lost 50% of visual ability. Zero is reached for 20/2000, a level at which it is difficult to speak about visual acuity, since 100x magnification is needed for detail recognition.
In theAMA Guides to the Evaluation of Permanent Impairment(5th and 6th edition) the VAS scale is used in the calculation of visual disability. A study has shown that the AMA calculations provide better ability estimates than scales that were used previously.
Logarithmic Progression
The above discussion already indicated why a logarithmic progression of letter sizes is desirable and leads to more consistent measurements. The following table compares the characteristics of ETDRS-type charts with previous charts with an irregular progression. Although the name “Snellen chart” should apply to any letter- or symbol-based chart, some authors use the term Snellen chartonly for charts with a non-logarithmic progression.
| Traditional charts | ETDRS type charts |
| The line-to-line steps are irregular, ranging from 14% (20/70 to 20/80) to 100% (20/100 to 20/200 to 20/400) | All line-to-line steps are equal 10√10, which is about a 4:5 ratio. |
| The number of letters per line varies. Thus the effect of missing one letter varies. | All lines have five letters. The effect of missing one letter is the same for all lines. |
| The spacing of letters and lines is dictated by the available space. Crowded letters are more difficult to recognize than widely spaced ones. | The letter-to-letter and line-to-line spacing is equal to the letter size. The relative crowding effect is the same for all lines. |
| Some letters are easier to recognize than others. This may result in easier and more difficult lines. | The difficulty of the letters used is taken into account. The average difficulty on each line is balanced. |
| Usually, only a single chart is used. This may lead to (voluntary or involuntary) memorization. | The ETDRS set consists of three charts to avoid memorization. |
| The shape of the chart is rectangular. | The shape is an inverted triangle. |
The evolution of letter charts fromSnellen’soriginal chart to theETDRS chart, which is considered today’s gold standard, has been discussed further on our Web site. Practitioners and researchers from all parts of the world turn to Precision Vision for“ETDRS” chartsand other high-quality vision testing tools.
The following table compares different visual acuity notations.
| US notation | 6 meter notation | Decimal notation | MAR | logMAR | VAS |
| 20/10 | 6/3 | 2.0 | 0.5 | -0.3 | 115 |
| 20/12.5 | 6/3.8 | 1.6 | 0.63 | -0.2 | 110 |
| 20/16 | 6/4.8 | 1.25 | 0.8 | -0.1 | 105 |
| 20/20 | 6/6 | 1.0 | 1.0 | 0.0 | 100 |
| 20/25 | 6/7.5 | 0.8 | 1.25 | 0.1 | 95 |
| 20/32 | 6/9.5 | 0.63 | 1.6 | 0.2 | 90 |
| 20/40 | 6/12 | 0.50 | 2.0 | 0.3 | 85 |
| 20/50 | 6/15 | 0.40 | 2.5 | 0.4 | 80 |
| 20/63 | 6/18 | 0.32 | 3.2 | 0.5 | 75 |
| 20/80 | 6/24 | 0.25 | 4.0 | 0.6 | 70 |
| 20/100 | 6/30 | 0.20 | 5.0 | 0.7 | 65 |
| 20/125 | 6/38 | 0.16 | 6.3 | 0.8 | 60 |
| 20/160 | 6/48 | 0.125 | 8.0 | 0.9 | 55 |
| 20/200 | 6/60 | 0.10 | 10.0 | 1.0 | 50 |
| 20/250 | 6/75 | 0.08 | 12.5 | 1.1 | 45 |
| 20/320 | 6/95 | 0.06 | 16 | 1.2 | 40 |
| 20/400 | 6/120 | 0.05 | 20 | 1.3 | 35 |
| 20/500 | 6/150 | 0.04 | 25 | 1.4 | 30 |
Related Terms: Snellen chart, Snellen eye chart, Snellen, Snellen eye test, Snellen visual acuity, Snellen acuity, Snellen vision chart, Snellen acuity, Snellen test
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March 21, 2014
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FAQs
How do you interpret Snellen score? ›
Recording Snellen Results
Top number equates to the distance (in metres) at which the test chart was presented (usually 6m), Bottom number identifies the position on the chart of the smallest line read by the 'patient'. Eg; 6/60 means the subject can only see the top letter when viewed at 6m.
Snellen charts display letters of progressively smaller size. "Normal" vision is 20/20. This means that the test subject sees the same line of letters at 20 feet that person with normal vision sees at 20 feet. 20/40 vision means that the test subject sees at 20 feet what a person with normal vision sees at 40 feet.
What is considered normal vision of a Snellen eye chart? ›The Snellen Chart uses a geometric scale to measure visual acuity, with normal vision at a distance being set at 20/20.
How do you read eye chart results? ›Normal Results
Visual acuity is expressed as a fraction. The top number refers to the distance you stand from the chart. This is often 20 feet (6 meters). The bottom number indicates the distance at which a person with normal eyesight could read the same line you correctly read.
The third line is equivalent to 20/40, it is the driver's test line.
What is 0.5 on the Snellen scale? ›You must also meet the minimum eyesight standard for driving by having a visual acuity of at least decimal 0.5 (6/12) measured on the Snellen scale (with glasses or contact lenses, if necessary) using both eyes together or, if you have sight in one eye only, in that eye.
What does a vision of 20 60 mean? ›If your vision is 20/60, this means that from 20 feet away you can only see letters that would normally be visible at 60 feet.
What does a 20 40 vision mean? ›A person with 20/40 vision sees things at 20 feet that most people who don't need vision correction can see at 40 feet. This means that they are nearsighted, but only slightly. A person with 20/40 vision may or may not need eyeglasses or contacts, and can discuss his or her options with a doctor.
What does 20 70 vision look like? ›The ratio measurement of vision describes visual acuity, or the sharpness of vision, at 20 feet from an object. For example, having 20/70 vision means that you must be at 20 feet to see what a person with normal vision can see at 70 feet.
What is the minimum vision required for driving? ›To acquire a driver's license in the State of Florida, a customer must pass a vision test to determine if they meet the mandatory minimum vision standards. The minimum visual acuity standard is 20/70 in either eye or both eyes together with or without corrective lenses (if vision cannot be improved).
What does 6 60 mean in an eye test? ›
The Snellen test for visual acuity
If you can only read the top line of the chart, then this would be written as 6 / 60. This means you can see at 6 metres what someone with standard vision could see from 60 metres away.
If this is the smallest line a person can read, the person's acuity is "6/12" ("20/40"), meaning that this person needs to approach to a distance of 6 metres (20 ft) to read letters that a person with normal acuity could read at 12 metres (39 ft).
What is the normal range of visual acuity? ›20/20 vision is normal vision acuity (the clarity or sharpness of vision) measured at a distance of 20 feet. If you have 20/20 vision, you can see clearly at 20 feet what should normally be seen at a distance.
What line on eye chart is 20 70? ›If the smallest print you can read is line 3 (T O Z) from 20 feet away while wearing your regular glasses or contact lenses, the doctor records your vision (or visual acuity) as 20/70 with best correction.
How many letters can you miss on a Snellen chart? ›In a regular Snellen chart, you can only miss 2 or less for each eye. If you miss more than 2 letters, you have to move up to the next line. Note: For more accurate results, eye doctors does not use Snellen chart. You used a special chart and you read the bigger letters first.
What is bad vision score? ›20/30 to 20/60, this is considered mild vision loss, or near-normal vision. 20/70 to 20/160, this is considered moderate visual impairment, or moderate low vision. 20/200 or worse, this is considered severe visual impairment, or severe low vision.
What is the normal eye vision number? ›Having 20/20 vision means you have normal or average, vision. Some people have vision that's better than 20/20, like 20/15 vision or 20/10 vision. This means that you can see something 20 feet away (like a line on an eye chart) that most people can see when they're 15 feet away (20/15) or 10 feet away (20/10).
Is 20 50 considered legally blind? ›If you're legally blind, your vision is 20/200 or less in your better eye or your field of vision is less than 20 degrees. That means if an object is 200 feet away, you have to stand 20 feet from it in order to see it clearly.
What does 6'5 mean in an eye test? ›– if your visual acuity is 6/5 you can see a line of letters from 6 metres , which someone with normal eyesight can only see when they are 5 metres away. If you can't see the 6/6 line without glasses, you might require spectacles. Your optometrist will place lenses in front of your eyes until the chart becomes clearer.
What is 0.1 vision? ›1 Moderate visual impairment. Visual acuity <= 0.3 > 0.1. 2 Severe visual impairment. Visual acuity <= 0.1 > 0.05. Blindness.
What does a vision of 6 7.5 mean? ›
For the Snellen chart, when assigning vision as the lowest line on which all letters were read correctly, all participants with vision of 6/7.5 or better could read a number-plate at 20 m, and no participant with worse than 6/36 was able to successfully read a number-plate (figure 1).
What does a visual acuity of 6 7.5 mean? ›For the Snellen chart, when assigning vision as the lowest line on which all letters were read correctly, all participants with vision of 6/7.5 or better could read a number-plate at 20 m, and no participant with worse than 6/36 was able to successfully read a number-plate (figure 1).
What does Snellen 6 12 mean? ›If this is the smallest line a person can read, the person's acuity is "6/12" ("20/40"), meaning that this person needs to approach to a distance of 6 metres (20 ft) to read letters that a person with normal acuity could read at 12 metres (39 ft).
What does 6 12 mean on an eye test? ›So, if your vision is 6/12 this means that you can see, at 6 metres, what a 'normal' person can see at 12 metres from the chart. That is, your central vision is half of normal.
What does 6'5 mean in an eye test? ›– if your visual acuity is 6/5 you can see a line of letters from 6 metres , which someone with normal eyesight can only see when they are 5 metres away. If you can't see the 6/6 line without glasses, you might require spectacles. Your optometrist will place lenses in front of your eyes until the chart becomes clearer.
What is 0.8 on the Snellen scale? ›There are several ways to express the measured visual acuity; we use the decimal as well as the 20/20-system. Decimal: The visual acuity can be expressed in decimal or percentage for example 0.8 (80%), 1.0 (100%), or 1.2 (120%).
What is the minimum vision required for driving? ›To acquire a driver's license in the State of Florida, a customer must pass a vision test to determine if they meet the mandatory minimum vision standards. The minimum visual acuity standard is 20/70 in either eye or both eyes together with or without corrective lenses (if vision cannot be improved).
What does 6 36 mean in an eye test? ›Normal distance vision is recorded as 6/6. In the USA, this is known as “20/20” vision (measured in feet). In practical terms, Jane's acuity of 6/36 means that, at a distance of 6m, she can read the row of letters that a fully sighted person can read at 36m.
What is the normal eyesight number? ›Having 20/20 vision means you have normal or average, vision. Some people have vision that's better than 20/20, like 20/15 vision or 20/10 vision. This means that you can see something 20 feet away (like a line on an eye chart) that most people can see when they're 15 feet away (20/15) or 10 feet away (20/10).
Does 6 12 vision need glasses? ›Someone with 6/12 vision may have normal vision without eyeglasses. Although it is rare. Whereas someone with 6/6 may need eyeglasses. So 6/6 does not mean perfect vision and they may need eyeglasses.
What does a 20 40 vision mean? ›
A person with 20/40 vision sees things at 20 feet that most people who don't need vision correction can see at 40 feet. This means that they are nearsighted, but only slightly. A person with 20/40 vision may or may not need eyeglasses or contacts, and can discuss his or her options with a doctor.
What does 6 4 mean in an eye test? ›A Va of 6/4 means that at 6m, the smallest letter you can see can be seen by somebody with '20/20′ vision at 4 metres. Hence, 6/4 acuity is better than 6/6. Some people therefore can see better than 20/20.
Does 6'9 vision require glasses? ›In any case, unaided vision of 6/9 is good enough to continue to function normally in most situations. Have your eyes examined by your ophthalmologist who could explain the reason for this and guide you further.
Does 6/18 vision require glasses? ›6 / 12 & 6 / 18 – It means you are able to read letters at 6 mtrs, what a normal person can read at 12 mtrs & 18 mtrs. This is also not a normal condition.
What does 6/24 mean in an eye test? ›Perfect vision in a Snellen test would equal 6/6, which means the person can see at 6 metres what they 'should be' able to see at 6 metres. 6/24 for example, means the person can see at 6 metres what a normally sighted person could see much further away, that is at 24 metres.
What does 6 60 mean in an eye test? ›The Snellen test for visual acuity
If you can only read the top line of the chart, then this would be written as 6 / 60. This means you can see at 6 metres what someone with standard vision could see from 60 metres away.
Many people refer to 'perfect' vision as '6/6' or '20/20' (US notation measured in feet), but this is not strictly true – these terms refer to 'average' vision. If you achieve a vision measurement of 6/6, this means that you can see at a distance of 6m what an average person also sees at the same distance.