What is 20/20 vision? What is 6/6 visual acuity?
Visual acuity 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 (normal), 6/9 (one line less than 6/6), 6/12 (2 lines less than 6/6) etc as the Snellen fraction
Visual acuity is a recording of the vision of the eye. 6/6 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.
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 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 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 of the spaces between the lines subtends one minute of arc.
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.
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 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 recognition 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
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, generally ranging in size from J10 (large print) to J1 (very small print).
Since then, there have been several modifications of the Jaeger chart (or “Jaeger card”) 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 number allows for simple deductions. For at any viewing distance, and at any level of the chart, a three line increase in VA is two times (2x) better vision, 6 lines is 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)||Decimal||Min angle of resolution (MAR)||LogMAR|
|6/150||0.04||25.0’=10 to power 1.4 =1.26 to power 14||1.4|
|6/120||0.05||20.0’=10 to power 1.3 = 1.26 to power 13||1.3|
|6/95||0.063||16.0’=10 to power 1.2 =1.26 to power 12||1.2|
|6/75||0.08||12.5’=10 to power 1.1 = 1.26 to power 11||1.1|
|6/60||0.1||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|
|6/19||0.32||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|
|6/9.5||0.63||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||1.25||0.8’=10 to power -0.1=1.26 to power -0.1||-0.1|
|6/3.8||1.6||0.63’=10 to power -0.2=1.26 to power-0.2||-0.2|
|6/3||2.0||0.5’=10 to power -0.3 =1.26 to power -0.3||-0.3|
0.4’=10 to power-0.4=1.26 to power -0.4
|Reading cm||Equivalent Power|
Magnification Ratio 40cm
|Magnification Ratio 25cm|
|Reading 6.3||Dioptres 16|
Magnification 6.3 at 40cm
|Magnification 4.0 at 25cm|
|LogMAR 40cm||logMAR 25cm||VA 40cm||VA 25cm|
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 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 explains the individual reports of vision quality.
Distance vision impairment is considered
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.
The smallest letters one can see in a line of letters (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.
Seeing 20/20 or not having baseline vivual acuity assessed as an occupational health and safety measure is wise in such industries as working with lasers
- Uncorrected refractive errors
- Age-related macular degeneration
- Diabetic retinopathy
- Corneal opacity
All the above are the major sources of global vision impairment and can comprimise 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 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.
An even poorer vision that this can be described as count fingers, hand movements only or light perception.
Near 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 ( 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.
HONEY BEES AND VISUAL ACUITY
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 minutesBirds 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.
WHAT VISION DOES A DOG HAVE?
Dogs’ eyes only have two cones. Dogs can see shades of yellow, blue and brown, as well as various hues of gray, black and white.Dogs are thought to be shortsighted with visual cuity of about 6/24
Due to their wide eye position they have a wider field of vision than humans and good motion receptors.Dogs can hear and smell much better than humans though.
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 are compared via calibrations of contrast. Edge contrast sensitivity is assessed by using circular patches containing edges with reducing contrast eg Melbourne Edge Test.
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.
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
20/200 = 1.0 logMAR
However, despite normal recording, an individual could still complain of blurry vision.
|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.
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 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:
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.
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.
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.