How to Calculate and Interpret Visual Acuity

Visual acuity is the number an eye exam gives for how small a detail your eyes can make out, and it is usually written as a fraction such as 20/20 or 6/6.1Webvision. How to Calculate and Interpret Visual Acuity The fraction compares your eyes with a standard eye at a fixed testing distance, so a bigger bottom number means you need larger letters. The same eyes can earn different scores depending on whether glasses are worn, which chart is used, and even how bright the room is, so one number is a snapshot rather than a fixed trait.

What the Numbers in 20/40 or 6/12 Mean

In the fraction (called the Snellen fraction), the top number is the standard testing distance, usually 6 metres or 20 feet.2Webvision. How to Calculate and Interpret Visual Acuity The bottom number is the distance at which the letters on the smallest line you read would be just readable by a standard eye.3Webvision. How to Calculate and Interpret Visual Acuity So 20/40 means you read at 20 feet what a standard eye reads at 40 feet. Metric charts say the same thing in different units: 6/12 is the same result as 20/40, because both fractions equal one half.

20/20, or 6/6, is the standard reference, not perfect vision, and many healthy young adults see better than that, at 20/15 for example. That result means you read at 20 feet what a standard eye needs to be 15 feet away to read.

On an exam slip, OD means right eye and OS means left eye, and each eye is tested separately. The letters cc or sc mean with or without your glasses. A plus after the fraction, as in 20/30+2, means you read that line and also got two letters on the next one; a minus, as in 20/30-2, means you missed two letters on that line. If the chart only goes to 20/400, worse vision may be recorded as counting fingers.

Charts are arranged so the big E at the top is the 20/200 line and the lines shrink as you go down. Your result is simply the smallest line you could read. If you can’t read the top line from the usual distance, the examiner may move you closer and record the shorter distance. A result of 10/200 means you could read the big letter only from 10 feet.

The top number isn’t always 20 or 6. Some rooms are too short for a full lane, so charts may be viewed in a mirror or shrunk so the letters still cover the correct angle.

A common misreading is that 20/40 means you have half your vision. It means detail has to be twice as large for you to see it.

A result worse than 20/20 doesn’t say why. Needing glasses, a cloudy lens, and retinal disease can all give 20/40, so the number is a prompt for further testing rather than a diagnosis. If your slip shows a low number without glasses, the usual next step is a refraction, where the examiner tries lenses to see whether your score improves.

How the Score Is Calculated

Letters on a chart are built on a fixed angle. On a standard-sized line, a whole letter covers 5 minutes of arc at your eye, and each of its strokes covers 1 minute, with 1 minute being one sixtieth of a degree.4Webvision. How to Calculate and Interpret Visual Acuity5Wikipedia. Visual acuity For the 20/20 line viewed at 20 feet, that works out to letters roughly the size of 43-point type.6Wikipedia. Visual acuity The gap you must see in a letter such as an E or a C is one fifth of the letter’s height.7Wikipedia. Visual acuity

Because the angle is fixed, the physical size changes with distance. A letter that is correct at 20 feet would be far too small if you stood twice as far away. A line labelled 20/40 is simply built with letters twice the size, covering 10 minutes of arc, with each stroke covering 2 minutes.

The fraction converts into other formats:

Take 6/18 as an example. Dividing 6 by 18 gives a decimal of about 0.33. Flipping the fraction gives an angle of 3 minutes of arc, and the log of 3 is about 0.48, so the logMAR score is roughly 0.5. To go from feet to metres, keep the ratio the same, so 20/60 equals 6/18.

Each doubling of the smallest visible detail adds about 0.3, which is why 20/20, 20/40 and 20/80 sit at 0.0, 0.3 and 0.6. A score of -0.1 corresponds to 20/16, better than the standard reference.

Each line on a logMAR chart is 0.1 logMAR and usually has five letters, so each letter read is worth 0.02 logMAR. That lets the score be recorded more finely than a whole line. Equal steps on these charts represent equal changes in vision, while on an older-style chart the jumps between lines are uneven, especially near the bottom. That is why researchers and eye specialists track changes over time in logMAR.11Eye. A history of visual acuity testing and optotypes

Is Your Result Normal

The World Health Organization counts acuity between 20/60 and 20/200 in the better eye, with the best possible correction, as low vision.12AMA Journal of Ethics. Legal Vision Requirements for Drivers in the United States It counts acuity worse than 20/400, or a field of view narrower than 10 degrees, as blindness.13AMA Journal of Ethics. Legal Vision Requirements for Drivers in the United States A narrowed field, below 20 degrees, can also qualify as low vision even when the person reads letters well.14AMA Journal of Ethics. Legal Vision Requirements for Drivers in the United States

In the US, legal blindness is the government’s yardstick for benefits, rehabilitation, schooling, devices, and tax programs.15The American Foundation for the Blind. Low Vision and Legal Blindness Terms and Descriptions It says little about what a person can actually see day to day.16The American Foundation for the Blind. Low Vision and Legal Blindness Terms and Descriptions It is generally 20/200 or worse in the better eye with correction, or a field of 20 degrees or less. Being legally blind does not mean seeing nothing; many people have useful sight and read large print or use screens. Driving rules are a separate standard set by each state, so passing a licence test and meeting the legal blindness threshold are unrelated decisions.

Categories rest on the better eye because most daily tasks, such as walking or reading, are done with both eyes open and the stronger eye carries most of the load. Someone with one very poor eye and a good other eye is generally not low vision by these definitions, even though depth perception and the side field may be affected.

Your score without glasses mostly reflects your need for glasses. Your score with the best possible lenses, called best-corrected visual acuity (BCVA), reflects the health of the eye. So if your score with glasses is reduced, it points toward a problem that glasses cannot fix.

At 20/40 you can usually meet the driving standard in many US states. Between 20/60 and 20/200, ordinary glasses no longer fully fix the problem, and people may turn to stronger magnifiers, larger print, or brighter task lighting.

An exam should be booked when your score with glasses is worse than 20/40. A sudden drop in vision in one eye needs prompt care. Ask your eye doctor whether a low score comes from needing glasses or from eye disease.

Why Your Score Can Change From Test to Test

On average, people score about 6.5 letters better on a modern logMAR-style chart than on a classic Snellen chart.17PubMed Central. Prospective Evaluation of Visual Acuity Assessment: A Comparison of Snellen Versus ETDRS Charts in Clinical Practice (An AOS Thesis) The gap grows as vision gets worse, reaching about 10 letters, or two lines, below 20/200.18PubMed Central. Prospective Evaluation of Visual Acuity Assessment: A Comparison of Snellen Versus ETDRS Charts in Clinical Practice (An AOS Thesis) So compare results only from the same kind of chart.

Lighting matters too. Scores are better in a lit room than a dark one, in eyes with or without glasses.19Australian Orthoptic Journal. The effect of room illumination on visual acuity measurement20PubMed Central. The effect of central and peripheral luminance on visual acuity Pupil size plays a part because the best pupil size balances blur from the eye’s optics against the eye’s sensitivity to light.21PubMed. Effect of Target Luminance on Optimum Pupil Diameter for Presbyopic Eyes When the eye is slightly out of focus in bright light, optical blur controls how sharp vision is.22PubMed. Interaction of aberrations, diffraction, and quantal fluctuations determine the impact of pupil size on visual quality

Screens add their own problem. On some tablets held at 40 cm, one pixel is larger than the smallest detail a healthy eye can see.23PubMed Central. Validation of visual acuity applications for teleophthalmology during COVID-19 Home vision apps may therefore distort the smallest letters, which makes them unreliable at the 20/20 level.

A pinhole check can show whether focus is the issue. Looking through a tiny hole blocks blurred light from the edges of the lens, acting like a very small pupil so only a narrow beam enters the eye. If vision improves, the problem is probably one that glasses can fix. If it does not, the cause may be in the eye’s health, since a pinhole doesn’t help where the retina or nerve is damaged. A better result doesn’t prove glasses are the whole answer either, because some cataracts also improve a little. To try it at home, poke a small hole in a card with a pin, hold it close to one eye with the other covered, and compare the same print with and without it.

To get a fair reading, bring the glasses or contacts you normally use so the examiner can record your result with them and then compare it with the new lenses. Keep guessing on the hardest line, because the score is based on how many letters you get right, and a few correct guesses can legitimately move you up a line. Squinting, tiredness, or dry eyes can briefly change the result, so blinking and resting beforehand gives a more typical reading.

How Children and Older Adults Are Measured

Babies too young to name letters are tested with striped cards, by watching which side they look at.24PubMed Central. Reference values for the Teller Acuity Cards II (TAC II) in infants and preverbal children, a meta‐analysis Children aged 3 to 4 score better on a tumbling E chart than on a broken-ring C chart.25PubMed Central. Development of visual acuity in preschool children as measured with Landolt C and Tumbling E charts By ages 5 to 6, the two charts agree and children’s scores approach adult levels.26PubMed Central. Development of visual acuity in preschool children as measured with Landolt C and Tumbling E charts So a preschooler’s result should be judged against norms for that age and that chart.27PubMed Central. Development of visual acuity in preschool children as measured with Landolt C and Tumbling E charts

In the tumbling E test the child points a hand or fingers in the direction the E opens, which avoids needing letter names. Pictures or matching cards can be used instead, since a child who knows the shapes can point to a match. Young children tire quickly, so a single weak result might reflect attention more than eyesight, which is why a repeat test is common.

On a parent’s screening slip, a 3-year-old scoring 20/40 may be normal for their age. Repeat checks on the same chart give a better trend. Amblyopia, or lazy eye, causes poor central vision in one eye and is treated best early.

Healthy eyes lose about 0.3 logMAR, roughly three lines, between middle age and 88.28PubMed. The decline in visual acuity in elderly people with healthy eyes or eyes with early age-related maculopathy in two Scandinavian population samples The pupils shrink, the lens clouds, and retinal and brain cells change with age.29PubMed Central. Vision through Healthy Aging Eyes Aging also reduces contrast sensitivity, slows adjustment to the dark, and reduces near focus.30PubMed Central. Vision through Healthy Aging Eyes

Smaller pupils let in less light, so older adults often need brighter light for reading even when their score looks fine in a well-lit clinic. A clouding lens scatters light and can cause glare and lowered contrast, which is why cataract surgery often helps people who still read 20/30 on a chart. At 70, a modest slip toward 20/30 can be normal aging. A large drop or distortion suggests disease, and a sudden change in one eye is not part of normal aging. Near vision decline from presbyopia is separate from distance acuity.

Reading vision is recorded on near charts such as Jaeger or N-notation charts.31Wikipedia. Near visual acuity The M scale gives the distance in metres at which a lowercase x covers 5 minutes of arc.32Wikipedia. Near visual acuity

What Visual Acuity Does Not Measure

Acuity tests sharp black letters on white. Contrast sensitivity, meaning the ability to see faint gray shapes, can be impaired even when the acuity score looks near normal.33PubMed Central. Relationship Between Acuity and Contrast Sensitivity: Differences Due to Eye Disease For the same acuity loss, glaucoma, macular degeneration, and especially retinitis pigmentosa damage contrast far more than cataract does.34PubMed Central. Relationship Between Acuity and Contrast Sensitivity: Differences Due to Eye Disease Contrast tests can pick up some vision damage that standard charts miss.35PubMed. Visual contrast sensitivity testing: a comparison of two F.A.C.T. test types

Contrast is tested with letters or stripes that fade from dark to pale gray, and a person may read the high-contrast chart well and still miss the faint ones. Real life is mostly low contrast, such as a gray curb on a gray road, a face in fog, or a step in a dim hall, so trouble can appear while acuity stays normal. Signs include trouble seeing curbs or faces in dim light despite 20/20, glare from headlights at night, or bumping into objects off to the side, which suggests field loss. Ask for a contrast or field test if these occur.

The chart tests only the small central area you aim at the letters, so someone with tunnel vision can read 20/20 and still have a very narrow field. Peripheral loss often goes unnoticed because the brain fills gaps. A separate visual field test, where you press a button when you see lights, is how it is found, and that is why the World Health Organization has field criteria alongside acuity.

Finally, biology sets a ceiling. The spacing of the eye’s central light-sensing cells sets the finest detail it can capture.36eLife. Human foveal cone photoreceptor topography and its dependence on eye length Even a perfect optical system could not read finer detail than that allows, so a 20/10 result is close to the limit for some healthy eyes. Glasses help up to a point but cannot push anyone beyond what the retina can pick up.