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

Visual acuity is a recording of the vision of the eye.

Standardised charts mean different clinics can record a result that should not vary depending upon where  conducted. So the visual acuity meaning can be expressed reliably as an acuity definition the test is performed with standard charts. However effects such as pupil size, contrast, optotype used,refractive error can cause variations.

In a visual acuity test 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 and is often thought of perfect vision in eye testing.(more by the lay person as even better acuities are possible).

Whereas eg 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.
A visual acuity test is 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 central area called the fovea, 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 the peripheral vision. However, a mere 20 degrees away from your sightline, your visual acuity is about 1/10th of what it is at the centre which can be measured by various eye charts.

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

 

Eye test charts

illiterate
visiual acuity
afl glasses

The most commonly used eye chart is known as the Snellen chart. It usually shows 11 rows of capital letters. The first line has one very large 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 eye sight 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 of the spaces between the lines subtends one minute of arc.

When using the so called LogMAR chart, visual acuity is scored with reference to the logarithm of the Minimum Angle of Resolution as opposed to the correlated  Snellen notation used for the eye and vision. 

20/20 vision or 6/6 vision means resolution of 1 arcmin as per other optotypes. Field tests are also an important indicator for eye health of the optic nerve and visual pathways

 

relative size single acuity
Landolt C letter
Landolt C Symbols
Lea symbols
Lea Symbols

Vision and resolution

The smallest letters one can see in a line of letters on a visual acuity chart (the most common form of visual acuity measurement) relies on the ability to see 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 eg  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.

The resolution of a human eye  varies from person to person, of course. If you had perfect vision, your acuity define resolution would be about 0.6 arcminutes  (for comparison, the full Moon on the sky is about 1/2 a degree or 30 arcmin across).In eye disease as well as in 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 a vital unit of measurement 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.

3/60 vision means the eye is capable of resolving the 6/60 letter size but at the closer distance of 3 metres.
1/60 vision means the eye is capable of resolving the 6/60 letter size but at the closer still distance of 1metre.
Worse vision that this can be described as count fingers,hand movements only or light perception.

Near visual acuity is 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  standardized for size or test distance. T

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

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 verus 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 explains the individual reports of vision quality.

The ability to see is affected by many factors especially focusing  error conditions like near-sightedness, farsightedness or astigmatism let alone a myriad of eye diseases.
The ability for a person to read or  see the last line on the visual chart is recorded (at 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  
 

Tables exist for for equivalent Snellen distance acuities and point sizes for Times New Roman font ( In 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 having a serious eye condition having  their  visual result  expressed in the vision test  means it can be benchmarked.

corrected actuity chart
 
 

Honey Bees and acuity

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

Humans base their color combinations on red, blue and green, while bees base their colors on ultraviolet light, blue and green
Birds of prey such as eagles have 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. 
Humans can only see 180-degrees wheras predatory birds have nearly 360 degrees as their eyes are angled away from the midline of the face
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

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.
Instead of the letters getting smaller on each successive line, it is the contrast of the letters (relative to the chart background) that decreases with each line. Contrast sensitivity has been shown to be reduced in patients with glaucoma, diabetic eye disease, and cataracts.High and Low Contrast Visual acuity – Visual acuity is compared through calibrations of contrast.Edge contrast sensitivity is assessed using circular patches containing edges with reducing contrast eg  Melbourne Edge Test.

contrast vision

Some aboriginals in  tests have showed  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. Sometimes sine-wave gratings which vary by spatial frequency can be employed

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 : Night time 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.

 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 
Blind Pension entitlements

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