What is 20/20 vision? What is 6/6 visual acuity?

A visual acuity test is a recording of the vision of the eye. The ability for a person to read or see the last line on the visual chart is recorded (at a test distance of 20 feet or 6 metres) and defined by terminology 6/6 vision (standard), 6/9 (one line less than 6/6), 6/12 (2 lines less than 6/6) etc. as the Snellen fraction.

optometrist measuring visual acuity
acuity letter size
Acuity 6 meters or 20 feet

Visual acuity is a recording of the vision of the eye6/6 Acuity is equivalent to 20/20 Acuity for testing distance of 6 meters or 20 feet

EYE TEST CHARTS

Standardised charts mean different clinics can record a result that should not vary depending upon where it is conducted.  Furthermore, so the result can be expressed reliably as an individual’s corrected or uncorrected acuity by definition, the test is performed with standard charts. 

However, effects such as pupil size, contrast, optotype used, or a refractive error can cause variations.

If an observer has  6/6 visual acuity (or 20/20 vision ), the 6/6 size letters can be resolved at 6 metres (or 20 feet being 6 metres) normally using a Snellen eye chart in an eyesight test during the standard eye test is often commonly thought of as standard ideal vision benchmark.

Even better vision can be recorded, though.

Landolt C letter for visual acuity
Landolt C Symbols
Lea symbols for visual acuity
Lea Symbols

The most commonly used eye chart is known as the Snellen chart. It usually shows eleven rows of capital letters. The first line has one extensive letter. Each row after that has increasing numbers of letters that are smaller in size.

Dutch eye doctor Hermann Snellen developed the Snellen eye chart in the 1860s. He was a colleague of Dr Fransiscus Donders, who began diagnosing vision problems by asking people to look at a chart on a wall and tell him what they could see with each eye while the other was occluded.

The direction of the E discriminated at different sizes is one type of eye test chart used in eyesight tests. The minimum angle of resolution is one minute of arc is ‘normal’ is by convention.

At this distance, the letters representing “normal” or 6/6(metres) or 20/20(feet) acuity subtend an angle of five minutes of arc, and the thickness of the lines and the spaces between the lines subtends one minute of arc.

Whereas, e.g. 20/40 or 6/12 vision (being poorer vision than 6/6 or 20/20 ) means only the larger 6/12 letter can be resolved at the 6-metre testing distance. Someone with so-called 6/6 vision could resolve the 6/12 letter from a distance of 12 metres whereas
20/10 vision or 6/3 vision (better vision) means the letter can be seen only at 3 metres by the 6/6 limited person versus 6 metres for this better standard.
20/100 means this letter can be seen at 100′ by the 20/20 capable person but only at 20′ by this criterion for the poorer visioned person.
20/15 means this letter can be seen at 20′ by the 20/15 capable person but only at 15′ by the poorer visioned person by comparison.
Such readings are fundamental in an eye exam and can be performed with or without glasses or contact lenses.
Light enters our eyes and falls upon the retina. It is then converted into electrical impulses that the brain perceives as images. Only a small part of your retina, the fovea’s central area, can generate a high-resolution image. This is why we need to look directly at something, to see detail.
The rest of the retina lacks detail, but it contributes by adding peripheral vision. However, a mere 20 degrees away from your sightline, your recognition is about 1/10th of what it is at the centre, which various eye charts can measure.

Common notation examples:
monocular and binocular uncorrected distance (UDVA) at 6 m (20 ft), uncorrected intermediate (UIVA) at 80 cm (32 in), uncorrected near (UNVA) at 40 cm (16 in), distance corrected intermediate (DIVA), distance corrected near (DNVA), and best-corrected distance (BDVA) visual acuities

The original Jaeger eye chart was developed in 1867 and contained seven paragraphs, each printed in a successively smaller font size. The smallest paragraph you can read when holding the chart approximately 40 cm away determines your near visual acuity.

A Jaeger eye chart contains several blocks of successively smaller text, ranging from J10 (large print) to J1 (tiny print).

Since then, several modifications of the Jaeger chart (or “Jaeger card”) have been modified by different manufacturers. Unfortunately, modern Jaeger charts are not standardised, and the actual letter sizes on different Jaeger cards might vary slightly.

Sloan introduced the M-unit notation. The M-unit is the letter height that corresponds to a visual angle of 5 min of arc at a distance of 1 meter. The other print sizes derive from upwards and downwards multiplication, with decimal logarithmic steps of 100.1 starting from 1.0 for 1 meter.

The logMAR progression of numbers allows for simple deductions. At any viewing distance and any level of the chart, a three-line increase in VA is two times (2x) better vision, 6 lines are 4x better, 9 lines is 8x, and a 10 line increase is 10x. ( A geometric progression of letter-size or MAR (in multiples of 25% larger or 20% smaller) is a linear
progression in the logarithm of MAR.) Thus, a 4x telescope should give 6 lines of VA improvement.

Snellen acuity  (meters)   DecimalMin angle of resolution (MAR)LogMAR
 6/150    0.0425.0’=10 to power 1.4 =1.26 to power  14   1.4
 6/120   0.0520.0’=10 to power 1.3 = 1.26 to power 13   1.3
 6/95    0.06316.0’=10 to power 1.2 =1.26 to power 12    1.2
 6/750.0812.5’=10 to power 1.1 = 1.26 to power 11   1.1
 6/600.1 10.0’=10 to power 1.0 = 1.26 to power 10    1.0
 6/48 0.125  8.0’=10 to power 0.9 = 1.26 to power 9  0.9
 6/38 0.16  6.3’=10 to power 0.8 = 1.26 to power 8   0.8
 6/30 0.2 5.0’=10 to power 0.7 = 1.26 to power 7    0.7
 6/24 0.25 4.0’=10 to power 0.6 =126 to power 6    0.6
 6/190.32 3.2’=10 to power 0.5=1.26 to power 5   0.5
 6/15 0.4 2.5’=10 to power 0.4 =1.26to power 4 0.4
 6/12 0.5 2.0’=10 to power 0.3 =1.26 to power.3 0.3
 6/9.50.63 1.6’=10 to power 0.2 =1.26 to power 2 0.2
 6/7.5 0.8 1.25’=10 to power 0.1 =1.26 to power1    0.1
 6/6 1.0 1.0’=10 to power 0.0 =1.26 to power 0   0.0
 6/4.8 1.250.8’=10 to power -0.1=1.26 to power -0.1   -0.1
 6/3.8 1.60.63’=10 to power -0.2=1.26 to power-0.2 -0.2
 6/3 2.00.5’=10 to power -0.3 =1.26 to power -0.3-0.3
 6/2.4 2.5

0.4’=10 to power-0.4=1.26  to power -0.4  

-0.4
Reading   cmEquivalent Power

Magnification Ratio 40cm

Magnification Ratio 25cm
502

0.8

0.5
402.51.00.63
323.21.250.8
254.01.61.0
205.02.01.25
166.252.51.6
12.58.03.22.0
10104.02.5
812.55.03.2
Reading 6.3Dioptres 16

Magnification 6.3  at 40cm

Magnification 4.0 at 25cm 
5.0208.05.0
4.02510.06.3
3.23212.58.0
2.54016.010.0
2.05020.012.5
N print

M units

LogMAR 40cmlogMAR 25cmVA 40cmVA 25cm
80101.41.66/1506/240
6481.31.56/1206/190
506.31.21.46/956/150
4051.11.36/756/120
3241.01.26/606/95
253.20.91.16/486/75
202.50.81.06/386/60
162.00.70.96/306/48
12.51.60.60.86/246/38

N print

10

M units

1.25

LogMAR 40cm

0.5

LogMAR 25cm

0.7

VA 40cm

6/19

VA 25cm

6/30

81.00.40.66/156/24
6.30.80.30.56/126/19
5.00.630.20.46/9.56/15
4.00.50.10.36/7.56/12
3.20.40.00.26/66/9.5

Why do many people have less than 20/20 or 6/6 vision?

Corrected visual acuity

This is a measure of your central vision, not peripheral vision or visual fields. If you do not need a spectacle power to read the line, it is described as unaided versus aided in cases where spectacles are required. Some patients cannot read the last 6/6 line due to limitations in their optical system or health. Each eye has distinctive higher and lower order aberrations which explain the individual reports of vision quality.

The ability to see is affected by many factors especially focusing on error conditions like near-sightedness, farsightedness or astigmatism, let alone many eye diseases.
The ability for a person to read or see the last line on the visual chart is recorded (at a test distance of 20 feet or 6 metres) and defined by terminology 6/6 vision (normal), 6/9 (one line less than 6/6), 6/12 (2 lines less than 6/6) etc. as the Snellen fraction. 
Distance vision impairment is considered.
Mild – presenting visual acuity worse than 6/12
Moderate – presenting visual acuity worse than 6/18
Severe – presenting visual acuity worse than 6/60
Blindness – presenting visual acuity worse than 3/60
Near vision impairment: presenting near visual acuity worse than N6 or M.08 with existing correction.

Visual resolution

The smallest letters one can see in a line of letters  (the most common form of visual acuity measurement) rely on seeing two sources very close together, which is called resolution. 

For someone with 6/6 acuity, they can resolve 1 minute of arc (in a letter 5 minutes of arc high).  Distance visual acuity means, in these cases, the distance isn’t relevant, e.g. at 6m (6,000mm), the height of a 6/6 letter is 8.73mm; at 60m, it has to be 87.3mm and at 600m, 873mm etc. 
At  6m, the resolution is 1 minute of arc (letter height is 5 mins of arc), and thus 8.73/5=1.75mm is the gap size for a 6/6 letter.

Seeing 20/20 or not having baseline visual acuity assessed as an occupational health and safety measure is wise in such industries as working with lasers. 

Uncorrected refractive errors

Cataract

Age-related macular degeneration

Glaucoma

Diabetic retinopathy

Corneal opacity

Trachoma

All the above are significant sources of global vision impairment and can compromise your acuity. The resolution of a human eye varies from person to person, of course. If you had perfect vision, your acuity defines resolution would be about 0.6 arcminutes (for comparison, the full Moon on the sky is about 1/2 a degree or 30 arcmins across).In eye disease and cases of refractive error measurement for glasses or contact lenses, the visual acuity is recorded at 20 feet or 6 metres, usually using the Snellen chart as an integral unit of measure and comparison. 

At about 300 pixels per inch, an iPhone display at 10 to 12 inches away from your eyes is the limit of the human retina to differentiate the pixels.

This means the pixels in this instance are so small that they exceed your eye’s ability to detect them. Pictures at that resolution are smooth and continuous, and not pixellated.

Whilst vision is registered if an object of whatever size or  distance  transfers a photon to a retinal cell,visual acuity is limited by the spacing between the cones and rods packed onto the retina. The optics of the eye prevent photons from alighting upon a photoreceptor cell as well.
Theoretically, studies have shown, 120 pixels per degree of arc is the maximin resolution possible.

Close readings are typically tested with a reading card to assess a subject’s ability to accommodate. Ratings such as Jaeger scores (designated J1, J2…) are sometimes used for near acuity but often are not standardised for size or test distance. 

Tables exist for equivalent Snellen distance acuities and point sizes for Times New Roman font ( Typography, the point is the smallest unit of measure.)

Newsprint is typically between 10- and 14-point or between J7 and J10.

Other discrimination tests may involve using simple pictures of houses, flowers, animals or other objects.

These kinds of charts can be used with young children or people who cannot read. Not only can a distance vision measurement be performed but also at near. Monocular and binocular readings can be recorded. A person with normal vision or a severe eye condition having a visual result expressed in the vision test can be benchmarked.

Magnification loupes are often used in specific industries. 

The terms mental acuity and cognitive acuity refer to the same functions and abilities of the human brain. Visual acuity refers to the vision capability of the eye determined by subjective or objective means.

The term 20/20 vision means you can see clearly at 20 feet what should typically be seen at that distance. 

15/15 vision means usual sharpness of vision at 15 feet, just as 20/20 indicates usual sharpness of vision at 20 feet, just as 40/40 vision means average sharpness of vision at 40 feet. For consistency, doctors of optometry in the United States use 20 feet as the standard for measuring the sharpness of vision, whereas in Australia, a 6-meter reference standard applies

The terms mental acuity and cognitive acuity refer to the same functions and abilities of the human brain. Many factors affect visual acuity. Vision conditions, like nearsightedness, farsightedness or astigmatism, or eye diseases affect the clarity of vision. If your vision is less than 20/20, or 6/6, an eye examination will uncover reasons, e.g. defects in your health or in vision skills, including peripheral awareness or side vision, eye coordination, depth perception, focusing ability and colour vision.

VISUAL ACUITY AND CATARACTS

Cataract and vision

Some criteria suggested for cataract removal is in cases where reduced visual acuity is 6/12 or worse, or disabling glare or contrast sensitivity is found.

Cataract Clinical Care Standards aim to address current variation in rates of cataract surgery between the public and private sectors, urban and remote areas, and for Aboriginal and Torres Strait Islander people compared with non-Indigenous Australians.A seven fold variation in such rates was found in the Second Australian Atlas of Healthcare Variation (2017).


Visual acuity measured on the widespead use of the Snellen chart is biased to high contrast visual acuity and a patient with a clinically significant cataract can actually register better than 6/12 visual acuity accordingly.The visual function needs to be considered over different contrast settings and pupil sizes.
Snellen acuity may be considered one part of an assessment to prioritise surgery lists, but 6/12 is somewhat contentious.
Untreated cataract patients are 8x more likely to suffer falls and 3x to fracture a bone.

HONEY BEES AND VISUAL ACUITY

flying bee

Honey bees can clearly see objects that are as small as 1.9°. In terms of the smallest object, a bee can detect, but not clearly, this works out to be about 0.6°  (about 6/24). At 6 meters, an object 2 degrees high would be equivalent to around 6/150 (125 minutes total height) or a MAR of 25 minutes. Birds of prey such as eagles have a high-definition vision of the order of 6/1.5 or so. Many birds of prey see greater degrees of colour shades and contrasts, including ultraviolet (UV) light. If you have 6/30 vision, you must be as close as 6 meters to see what a person with normal vision can see at 30 meters. 3/60 vision means the eye can resolve the 6/60 letter size but at a closer distance of 3 meters. 1/60 vision means the eye can resolve the 6/60 letter size but at the closer still distance of 1 meter. An even more flawed vision can be described as count fingers, hand movements only or light perception.

WHAT VISION DOES A DOG HAVE?

Dog vision

Humans can make visual discrimination of objects from roughly a six-fold distance compared to dogs. Humans have better three different colour receptors red, blue, green, versus only blue and yellow for dogs.
The brightness discrimination of dogs is about 2 times worse than that of humans. If a human and a dog were looking at black and white stripes, the human would see the black and white more clearly. For the dog, the black and white lines would blur together more. (when acuity units referred to “cycles per degree.” Each “cycle” is the same as a “line pair”—namely, one black/white pair taken together.  per degree. ( 20/20 and 6/6 vision correspond to 30 cycles per degree.)

Contrast sensitivity

contrast vision

Correct identification of the orientation of the edges on the patches provides a measure of contrast sensitivity.
Pelli-Robson test measures contrast sensitivity by discerning letters at the minimum contrast.

An object can only be distinguished from its surroundings if there is brightness or colour difference – called contrast – between them. A minimum contrast, called the contrast threshold, is required to detect an object. The modulation transfer function (MTF) is the spatial frequency response of an imaging system or a component. It is the contrast at a given spatial frequency relative to low frequencies.
Instead of the letters getting smaller on each successive line, the contrast of the letters (relative to the chart background) decreases with each line. Contrast sensitivity is reduced in patients with glaucoma, diabetic eye disease, and cataracts.  High and Low Contrast Visual acuity is compared via calibrations of contrast. Edge contrast sensitivity is assessed using circular patches containing edges with reducing contrast, e.g. Melbourne Edge Test. the human contrast sensitivity function means that finer spacings are perceived with much lower clarity.

Humans base their colour combinations on red, blue and green, while bees base their colours on ultraviolet light, blue and green.

As humans, age accommodation for near purposes becomes less effective, requiring optical aids, but even distance viewing can often be improved by optical correction or lighting improvement.  

Contrast Sensitivity is the minimum amount of contrast required to detect sine-wave gratings at different spatial frequencies.
Contrast sensitivity is the reciprocal of the contrast threshold.
Low threshold = high contrast sensitivity
High threshold = low contrast sensitivity
One’s contrast sensitivity can be plotted for each spatial frequency.
The result is the contrast sensitivity function (CSF).

Grating acuity can be predicted based on photoreceptor diameter and spacing as an unstimulated photoreceptor must lie between two stimulated photoreceptors.

Ideally, CS tests have, e.g. an illuminated wall chart with a measured standard luminance and surround lighting. An example is the Melbourne Edge Test. There is also a chart with sinusoidal gratings at different orientations and low contrast letters of different sizes.

The Melbourne Edge test is an illuminated wall chart with a measured standard luminance and surrounds lighting.

Even an Ipad-based letter contrast sensitivity tests similar to the well-established Pelli-Robson Chart, the Mars Letter Contrast Sensitivity Test and Rabin’s Small Letter Contrast Sensitivity Test have been developed.

The test presents four lines of five letters on each page. The contrast of each successive line decreases in 0.1 log unit steps.

Six sequences represent three sizes: 7.5 M, 3.8 M and 1.9 M. At 1 meter, these are equivalent to 20/150, 20/75 and 20/38, respectively. In terms of spatial frequency, these correspond to 4, 8 and 16 cycles per degree. Users can adjust the test distance to suit their needs. Five letters are on each line, and the contrast decreases by 0.1 log units per line. The values at the top of the page are log contrast sensitivity (= –log10contrast). Thus 1.0 represents 10% contrast and 1.5 is around 3% contrast.

Subjects read the letters, starting with high contrast until no letters on a given page are read correctly.

The letters on the 20/20 row will subtend 5 minutes of arc vertically. The actual size of the letters at different distances are found using some simple trigonometry:

Letter height = Tan(5/60) * Viewing distance

So at 6 metres the 20/20 letters will measure 6000 x 0.00145 = 8.727mm

At 3 metres they will be half the size and at 1 metre 1/6th of the size. The size of the other letters can be found by simply multiplying the size of the 20/20 letters by the corresponding factor 20/40(x 2) 20/200 (x 10) etc. Font size converter tools allow for correct calibration of printed charts to be used at different distances if required.

LogMAR stands for Log of the Minimum Angle of Resolution. The Minimum Angle of Resolution is the angle between the limbs of the letters (most easily explained with a letter E).

By definition, a 6/6 letter subtends a total of 5 minutes of arc vertically and so if you think of a letter E, the gap between the limbs (the MAR) is 1 minute and the LogMAR is 0. A 6/60 letter is ten times larger, therefore, the MAR is 10 and the LogMAR= 1 etc.

LogMAR= Log (1/ Decimal VA)

Some aboriginals in tests have shown vision as high as 6/2 meaning the chart could be read 6 meters away as well as someone with “normal 6/6” vision could read it from 2 metres away.  Vision at this level requires that the eyes lack normal irregularities in shape and structure that would otherwise cause optical aberrations.  This resolution acuity is the smallest stimulus that can be resolved from a uniform background usually at high contrast and can be measured behaviorally or electrophysiologically.

LogMAR tests provide a better alternative
Equal  number of letters
Proportionate spacing
Equivalent letter difficulty
Regular line progression using logMAR units. Log10 minimum angle of resolution   Smaller number = better vision  ie

20/200 = 1.0 logMAR
20/20 = 0 logMAR
20/16 = -0.1 logMAR
Lines progress in 0.1 logMAR units

However,  despite normal recording, an individual could still complain of blurry vision.

 FeetMeter DecimalLogMar
 20/200 6/60 0.100   1.0 
 20/160 6/48 0.125  0.9
 20/125 6/38 0.16  0.8
 20/100 6/30 0.20  0.7 
 20/80 6/24 0.25  0.6
 20/63 6/19 0.32  0.5
 20/50 6/15 0.40  0.4
 20/40 6/12 0.50  0.3
 20/32 6/9.5 0.63  0.2
Three kinds of receptors designated the long-wavelength, medium-wavelength and short-wavelength cones are responsible for colour vision.
Diseases of the optic nerve can affect colour vision.
The severity of colour vision and visual loss can point to its cause among the variable colour blind associations.
They also do not measure other vision problems such as loss of peripheral (side) vision or depth perception stereopsis.
 20/25 6/7.5 0.80  0.1 
 20/20 6/6 1.00 0.0 
 20/16 6/4.8 1.25-0.1 
 20/12.5 6/3.8 1.60-0.2 
 20/10 6/3 2.00-0.3 

SPATIAL FREQUENCY TO logMAR

Visual acuity is also a measure of the spatial resolution of the visual system. It can also be measured in cycles(say of alternate black and white lines) per degree (CPD), which measures how much an eye can differentiate one object from another in terms of visual angles. The maximum visual acuity of the human eye 60 CPD.

Pattern glare symptoms usually include headaches or discomfort, particularly when reading.
Some readers find individually selected coloured overlay filters helpful for reading.
One possible mechanism for pattern glare is a hyperexcitability of cells in the visual cortex

Cycles per degree CPDSnellen equivalentLogMAR
0.1120/200020/2000
0.23320/160001.90
0. 4520/12001.80
0.6920/8001.65
1.3120/5001.40
1.8020/3551.25
3.6220/1600.90
7.2320/800.60
14.4620/400.30
28.9220/200

BLIND PENSION ENTITLEMENTS

blind-pension

If the best-corrected VA is less than 6/60, the patient is legally blind. 
If the visual field is essential ‘tunnel vision’, i.e. restricted to 10 degrees or less of arc around central fixation, the patient is legally blind 
if the patient has other problems, e.g. mobility issues or limb weakness, Disability Support Pension exists pending based on total equivalent disability.
With brain injury or further acquired brain damage, people can have a relatively small vision loss or significant loss requiring a comprehensive eye examination and workup. Visual acuity, nonetheless, should always be monitored. Diseases of the optic nerve affect colour vision earlier and more profoundly than other diseases. When the cause of visual loss is uncertain, colour vision and VA loss can point to such acquired eye disease.

“Legally blind” is more used in eligibility criteria for social services. A person with total blindness won’t be able to see anything but many people who meet the definition of legal blindness may be able to see light,colours and some functioning low acuity centrally or peripherally. If you have permanently reduced vision but retain some amount of your sight, you have low vision. About 15% are thought to be totally blind.

Many conditions can lead to low vision, including:

macular degeneration
glaucoma
cataracts
damage to the retina

Total blindness describes people with eye disorders who have no light perception (NLP). Total blindness can be the result of trauma, injury, or even conditions like end stage glaucoma or end stage diabetic retinopathy.

Congenital blindness
This description applies to people who are blind from birth. Some congenital eye conditions can develop during pregnancy and lead to blindness, while the causes of others are still unknown. Many people who have impaired vision don’t have additional sensory abilities that help them compensate for their vision loss but many do.

DARK ADAPTION

dark adaption

The lux is the SI derived unit of illuminance and luminous emittance, measuring luminous flux per unit area. It is equal to one lumen per square meter. In photometry, this is used as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface. The terms photopic, mesopic and scotopic refer to three ranges of human vision adaptation level.

Photopic: Refers to cone vision (most active medium and high light levels)
Scotopic: Nighttime or rod vision (providing peripheral vision, contrast and movement at low light levels)
Mesopic: This term refers to a range of human vision with both rods and cones active. There is no hard-line transition of dark adaption data for a particular eye.

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