A visual acuity test is one of the most common parts of an eye examination.
It measures how clearly you can see details at a specified distance and is often performed using a Snellen chart, the familiar chart containing rows of letters that become progressively smaller.
Visual acuity testing can help detect changes in vision, identify possible refractive errors and monitor the outcome of treatments such as cataract surgery.
However, visual acuity is only one part of a complete eye examination. A person can achieve 6/6 vision and still have an eye condition affecting other aspects of vision.
What Is Visual Acuity?
Visual acuity describes the ability of the visual system to distinguish fine detail.
Distance visual acuity is commonly tested at 6 metres, equivalent to approximately 20 feet.
This is why visual acuity results are often written as:
6/6
6/9
6/12
6/18
6/60
In countries using feet, the equivalent notation may be written as:
20/20
20/30
20/40
20/60
20/200
WHO describes visual acuity as a simple, non-invasive measure of the visual system's ability to distinguish high-contrast detail.
What Does 6/6 Vision Mean?
A result of 6/6 means that at a distance of 6 metres, you can see detail that a person with conventionally normal visual acuity would also be expected to see at 6 metres.
In feet, this is written as 20/20.
But 6/6 does not mean your eyesight is perfect in every possible way.
Visual acuity does not fully assess:
peripheral vision;
colour vision;
contrast sensitivity;
depth perception;
eye pressure;
retinal health;
optic nerve health; or
how well the eyes work together.
Someone can therefore have 6/6 vision and still have glaucoma, early retinal disease or another eye condition.
What Does 6/12 Mean?
A visual acuity of 6/12 means that from 6 metres away, you can see detail that a person with standard vision could see from 12 metres away.
Likewise, 6/18 means you need to be 6 metres from a target that someone with standard visual acuity could identify at 18 metres.
As the denominator becomes larger, distance visual acuity becomes poorer.
WHO uses visual-acuity thresholds as part of its classification of vision impairment.
Why Is a Visual Acuity Test Performed?
Visual acuity testing may be performed to:
establish a baseline record of vision;
investigate blurred or reduced vision;
screen for refractive errors;
monitor changes in eyesight;
assess vision before and after treatment;
evaluate the outcome of cataract or other eye surgery;
monitor known eye disease;
perform school or occupational screening; and
help determine whether further eye examination is required.
It is often one of the first measurements taken during an eye examination because it provides useful information quickly and non-invasively.
Who Can Perform a Visual Acuity Test?
Depending on the setting, visual acuity may be measured by:
an optometrist;
an ophthalmologist;
an ophthalmic nurse;
a trained vision technician;
another appropriately trained healthcare worker.
An ophthalmologist is a medical doctor specialising in eye diseases and surgery.
An optometrist is trained to examine vision, identify refractive problems and assess many eye conditions.
An optician mainly dispenses and fits spectacles and other optical devices and should not automatically be described as the professional responsible for diagnosing eye disease.
What Equipment Is Used?
Common visual acuity tools include:
Snellen charts;
LogMAR charts;
tumbling E charts;
symbol or picture charts for children;
electronic visual-acuity displays;
occluders;
pinhole occluders; and
appropriate spectacles or trial lenses.
Modern eye clinics may use illuminated or digital charts rather than traditional printed wall charts.
What Is a Snellen Chart?
The Snellen chart contains rows of letters of different sizes.
The largest letters appear at the top, while progressively smaller letters appear lower down.
The patient reads the smallest letters they can see reliably at the specified testing distance.
Each line corresponds to a level of visual acuity.
Although Snellen charts remain widely used, other charts such as LogMAR charts can provide more standardised measurements, especially in research and specialist clinical settings.
What Is a Tumbling E Chart?
A tumbling E chart uses the letter E facing different directions.
Instead of naming letters, the person indicates whether the arms of the E point:
up;
down;
left; or
right.
This can be useful for:
young children;
people who cannot read;
people unfamiliar with the alphabet; or
settings where language differences make letter charts unsuitable.
Picture or symbol charts can also be used for younger children.
How Is a Visual Acuity Test Performed?
The exact procedure can vary according to the chart and clinical setting.
Typically:
The person is positioned at the correct testing distance, commonly 6 metres for a standard distance chart.
The room and chart should have appropriate illumination.
One eye is covered without pressing on it.
The person reads progressively smaller letters or identifies symbols.
The smallest line read adequately is recorded.
The procedure is repeated for the other eye.
Vision may be measured with the person's usual spectacles or contact lenses.
If vision is reduced, additional testing may be performed, including a pinhole test or refraction.
Adult distance acuity is usually tested one eye at a time, with the fellow eye completely occluded.
Should You Guess During the Test?
It is usually acceptable to attempt letters even when you are not completely certain.
The purpose of the test is to determine the smallest level of detail you can reliably distinguish.
The old advice that a patient should make “no guesses” is therefore too rigid.
The examiner will determine how many letters need to be correctly identified for a line to count.
What Do Right Eye and Left Eye Mean on an Eye Record?
Eye-care professionals commonly use:
OD for the right eye
OS for the left eye
OU for both eyes together
A record might therefore say:
OD: 6/6
OS: 6/12
This means the right eye achieved 6/6 while the left eye achieved 6/12.
The record should also indicate whether the measurement was taken with or without correction.
What Does “With Correction” Mean?
“With correction” means the person was wearing their usual spectacles or contact lenses during testing.
Visual acuity may be recorded:
without correction, sometimes abbreviated as SC or unaided;
with correction, sometimes abbreviated as CC; or
with pinhole, often abbreviated as PH.
These distinctions are important because they help clinicians understand whether reduced vision may be related to refractive error.
What Is the Pinhole Test?
A pinhole occluder contains one or more very small holes.
Looking through the pinhole restricts peripheral light rays and allows more centrally directed light to reach the retina.
If visual acuity improves significantly through the pinhole, the reduced vision is often consistent with an uncorrected refractive error, such as:
short-sightedness;
long-sightedness; or
astigmatism.
American Academy of Ophthalmology guidance notes that improvement with pinhole testing suggests that an optical or refractive cause is likely.
What If Vision Does Not Improve With a Pinhole?
Failure to improve with a pinhole does not automatically diagnose a particular disease.
It suggests that reduced vision may not be explained simply by refractive error.
Possible causes could include problems involving:
the cornea;
lens;
retina;
optic nerve; or
other parts of the visual system.
Further examination is therefore needed.
The pinhole test is a screening clue, not a complete diagnosis.
What Happens If Someone Cannot See the Largest Letter?
If a person cannot identify the largest symbol at the normal testing distance, the examiner may reduce the distance.
For example, the person may be moved progressively closer to the chart.
The result must then be recorded using the actual testing distance.
If standard chart letters cannot be seen even at close distances, clinicians may assess whether the patient can detect:
counting fingers;
hand movements;
light perception; or
no light perception.
These assessments are used when vision is substantially reduced.
What Is Near Visual Acuity?
Distance acuity is not the same as near vision.
Near visual acuity assesses the ability to read or distinguish small details at a closer distance, commonly around 40 centimetres, depending on the test used.
Near vision becomes particularly important with age because of presbyopia, the gradual reduction in the eye's ability to focus on close objects.
WHO notes that near vision should be assessed separately from distance acuity.
What Can Cause Reduced Visual Acuity?
Reduced visual acuity can occur for many reasons.
Common causes include:
Refractive errors
These include:
myopia;
hyperopia;
astigmatism; and
presbyopia.
Uncorrected refractive error remains one of the leading causes of vision impairment globally.
Cataract
Clouding of the natural lens can cause gradual blurred vision, glare and reduced visual acuity.
Corneal disease
Scarring, infection or irregularities affecting the cornea may reduce vision.
Retinal disease
Conditions affecting the retina, such as diabetic retinopathy or macular disease, can affect central vision.
Optic nerve disease
Damage to the optic nerve can affect visual function.
Amblyopia
Amblyopia, sometimes called lazy eye, can cause reduced visual development in one eye during childhood.
Other neurological or ocular conditions
Vision can also be affected by conditions involving the visual pathways in the brain.
When Should You Have Your Eyes Checked?
You should consider an eye examination if you notice:
blurred vision;
difficulty reading;
difficulty seeing road signs;
frequent squinting;
persistent headaches associated with visual tasks;
double vision;
changes in vision;
difficulty seeing at night;
eye pain;
flashes or new floaters; or
a difference between the two eyes.
Children should also receive appropriate vision screening because they may not realise that one eye sees poorly.
When Is Sudden Vision Loss an Emergency?
A sudden or significant change in vision should not simply be handled by taking another visual acuity test at home.
Seek urgent medical assessment for symptoms such as:
sudden loss of vision;
sudden severe reduction in vision;
a dark curtain or shadow across the vision;
new flashes and many floaters;
severe eye pain with reduced vision;
eye injury;
chemical exposure to the eye; or
sudden neurological symptoms occurring with visual change.
Some causes of sudden visual loss are time-sensitive.
Is a Visual Acuity Test Painful?
No.
A standard visual acuity test is non-invasive and painless.
There is usually no special preparation required.
You should bring your usual spectacles or contact lenses if you use them.
Does Poor Visual Acuity Mean You Are Going Blind?
No.
Many people with reduced visual acuity simply have an uncorrected refractive error that can be improved with appropriate spectacles or contact lenses.
Other conditions may require medical or surgical treatment.
The visual acuity result itself tells us how clearly someone sees, but not necessarily why the vision is reduced.
That requires further assessment.
Can Someone Have 6/6 Vision and Still Need an Eye Examination?
Yes.
A Snellen result only measures one aspect of visual function.
A person can have apparently good central visual acuity and still have conditions such as:
glaucoma;
early diabetic retinopathy;
peripheral visual-field loss;
some retinal disorders; or
other eye diseases.
This is why visual acuity testing should not be confused with a complete eye examination.
Visual Acuity and Vision Impairment
WHO classifies distance vision impairment partly according to the visual acuity of the better eye.
Broadly, current categories include:
mild vision impairment: worse than 6/12 to 6/18;
moderate vision impairment: worse than 6/18 to 6/60;
severe vision impairment: worse than 6/60 to 3/60; and
blindness: worse than 3/60.
These categories are primarily used for clinical, epidemiological and public-health purposes and should not be used by individuals to diagnose themselves.
Why Regular Eye Care Matters
WHO estimated in 2026 that at least 2.2 billion people worldwide have near or distance vision impairment.
For at least one billion of them, the impairment could have been prevented or has yet to be appropriately addressed.
Refractive errors and cataracts remain among the leading causes.
Simple visual acuity testing can therefore play an important role in identifying people who need a more detailed eye examination.
Frequently Asked Questions
Is 6/6 perfect vision?
Not exactly.
6/6 represents a conventional standard for normal distance visual acuity, but it does not measure every aspect of eye health or vision.
Is 20/20 the same as 6/6?
Yes.
20/20 uses feet, while 6/6 uses metres.
Is 6/12 bad eyesight?
It represents reduced distance visual acuity compared with 6/6, but the clinical significance depends on the cause, whether correction is being worn and the person's circumstances.
Can spectacles improve visual acuity?
Yes, when reduced visual acuity is caused by a refractive error.
What does improvement with a pinhole mean?
It usually suggests that at least some of the reduced vision may result from refractive or optical error.
Can visual acuity testing diagnose glaucoma?
No.
Glaucoma can exist even when central visual acuity remains good. Diagnosis requires appropriate examination, which may include optic nerve assessment, visual-field testing and eye-pressure measurement.
Do both eyes need to be tested separately?
Yes.
Testing only both eyes together can hide reduced vision in one eye.
The Bottom Line
A visual acuity test measures how clearly you can see fine detail at a specified distance.
Results such as 6/6, 6/12 and 6/18 provide useful information about central vision, but they do not by themselves diagnose the cause of reduced eyesight.
A pinhole test may suggest whether refractive error contributes to blurred vision, but further examination is often required.
Most importantly, do not assume that achieving 6/6 means your eyes are completely healthy.
Visual acuity is an important starting point, not the whole eye examination.
If you notice persistent, worsening or sudden changes in vision, arrange an assessment with an appropriately qualified eye-care professional.
References
World Health Organization. (2026). Blindness and vision impairment.
World Health Organization. World report on vision.
American Academy of Ophthalmology EyeWiki. Visual Acuity Testing in Adults.
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