<unk>  scales 20 ft 10 ft 6 m 3 m Decimal LogMAR 20/1000 10/500 6/300 3/150 0.02 50 1.70 20/800 10/400 6/240 3/120 0.025 40 1.60 20/600 10/300 6/180 3/90 0.033 30 1.48 20/500 10/250 6/150 3/75 0.04 25 1.40 20/400 10/200 6/120 3/60 0.05 20 1.30 20/300 10/150 6/90 3/45 0.067 15 1.18 20/250 10/125 6/75 3/37 0.08 12.5 1.10 20/200 10/100 6/60 3/30 0.10 10 1.00 20/160 10/80 6/48 3/24 0.125 8 0.90 20/125 10/62 6/38 3/19 0.16 6.25 0.80 20/100 10/50 6/30 3/15 0.20 5 0.70 20/80 10/40 6/24 3/12 0.25 4 0.60 20/60 10/30 6/18 3/9 0.33 3 0.48 20/50 10/25 6/15 3/7.5 0.40 2.5 0.40 20/40 10/20 6/12 3/6 0.50 2 0.30 20/30 10/15 6/9 3/4.5 0.67 1.5 0.18 20/25 10/12 6/7.5 3/4 0.80 1.25 0.10 20/20 10/10 6/6 3/3 1.00 1 0.00 20/16 10/8 6/4.8 3/2.4 1.25 0.8 −0.10 20/12.5 10/6 6/3.8 3/2 1.60 0.625 −0.20 20/10 10/5 6/3 3/1.5 2.00 0.5 −0.30 20/8 10/4 6/2.4 3/1.2 2.50 0.4 −0.40 20/6.6 10/3.3 6/2 3/1 3.00 0.333 −0.48<unk>  is often measured according to the size of letters viewed on a Snellen chart or the size of other symbols, such as Landolt Cs or the E Chart. In some countries, acuity is expressed as a vulgar fraction, and in some as a decimal number. Using the metre as a unit of measurement, (fractional)<unk>  is expressed relative to 6/6. Otherwise, using the foot,<unk>  is expressed relative to 20/20. For all practical purposes, 20/20 vision is equivalent to 6/6. In the decimal system, acuity is defined as the reciprocal<unk> lue of the size of the gap (measured in arc minutes) of the smallest Landolt C, the orientation of which can be reliably identified. A<unk> lue of 1.0 is equal to 6/6. LogMAR is another commonly used scale, expressed as the (decadic) logarithm of the minimum angle of resolution (MAR), which is the reciprocal of the acuity number. The LogMAR scale converts the geometric sequence of a traditional chart to a linear scale. It measures<unk>  loss: positive<unk> lues indicate vision loss, while negative<unk> lues denote normal or better<unk> . This scale is commonly used clinically and in research because the lines are of equal length and so it forms a continuous scale with equally spaced intervals between points, unlike Snellen charts, which have different numbers of letters on each line. A<unk>  of 6/6 is frequently described as meaning that a person can see detail from away the same as a person with "normal" eyesight would see from 6 metres. If a person has a<unk>  of 6/12, he is said to see detail from away the same as a person with "normal" eyesight would see it from away. The definition of 6/6 is somewhat arbitrary, since human eyes typically have higher acuity, as Tscherning writes, "We have found also that the best eyes have a<unk>  which approaches 2, and we can be almost certain that if, with a good illumination, the acuity is only equal to 1, the eye presents defects sufficiently pronounced to be easily established." Most observers may have a binocular acuity superior to 6/6; the limit of acuity in the unaided human eye is around 6/3–6/2.4 (20/10–20/8), although 6/3 was the highest score recorded in a study of some US professional athletes. Some birds of prey, such as hawks, are believed to have an acuity of around 20/2; in this respect, their vision is much better than human eyesight. When<unk>  is below the largest optotype on the chart, the reading distance is reduced until the patient can read it. Once the patient is able to read the chart, the letter size and test distance are noted. If the patient is unable to read the chart at any distance, he or she is tested as follows: Name Abbreviation Definition Counting Fingers CF Ability to count fingers at a given distance. This test method is only used after it has been determined that the patient is not able to make out any of the letters, rings, or images on the acuity chart. The letters CF, and the testing distance, would represent the patient's acuity. For example, the recording CF 5 would mean the patient was able to count the examiner's fingers from a maximum distance of 5 feet directly in front of the examiner. (The results of this test, on the same patient, may<unk> ry from examiner to examiner. This is due more so to the size differences of the<unk> rious examiner's hands and fingers, than fluctuating vision.) Hand Motion HM Ability to distinguish whether or not there is movement of the examiner's hand directly in front of the patient's eyes. This test method is only used after a patient shows little or no success with the Counting Fingers test. The letters HM, and the testing distance, would represent the patient's acuity. For example, the recording HM 2 would mean that the patient was able to distinguish movement of the examiner's hand from a maximum distance of 2 feet directly in front of the examiner. (The results of the Hand Motion test are often recorded without the testing distance. This is due to the fact that this test is performed after the patient cannot "pass" the Counting Fingers test. At this point, the examiner is usually directly in front of the patient, and it is assumed that the Hand Motion test is performed at a testing distance of 1 foot or less.) Light Perception LP Ability to perceive any light. This test method is used only after a patient shows little or no success with the Hand Motion test. In this test, an examiner shines a pen light at the patient's pupil and asks the patient to either, point to the light source, or, describe the direction that the light is coming from (up, out, straight ahead, down and out, etc.). If the patient is able to perceive light, the letters LP are recorded to represent the patient's acuity. If the patient is unable to perceive any light, the letters NLP (No Light Perception) are recorded. A patient with no light perception in one eye is considered blind in the respective eye. If NLP is recorded in both eyes, the patient is described as having total blindness. Legal definitions<unk> rious countries have defined statutory limits for poor<unk>  that qualifies as a disability. For example, in Australia, the Social Security Act defines blindness as: In the US, the relevant federal statute defines blindness as follows: A person's<unk>  is registered documenting the following: whether the test was for distant or near vision, the eye(s) evaluated and whether corrective lenses (i.e. glasses or contact lenses) were used: * Distance from the chart * D (distant) for the evaluation done at . * N (near) for the evaluation done at . * Eye evaluated * OD (Latin oculus dexter) for the right eye. * OS (Latin oculus sinister) for the left eye. * OU (Latin oculi uterque) for both eyes. * Usage of spectacles during the test * cc (Latin cum correctore) with correctors. * sc: (Latin sine correctore) without correctors. * Pinhole occluder * The abbreviation PH is followed by the<unk>  as measured with a pinhole occluder, which temporarily corrects for refractive errors such as myopia or astigmatism. So, distant<unk>  of 6/10 and 6/8 with pinhole in the right eye will be: DscOD 6/10 PH 6/8. Distant<unk>  of count fingers and 6/17 with pinhole in the left eye will be: DscOS CF PH 6/17. Near<unk>  of 6/8 with pinhole remaining at 6/8 in both eyes with spectacles will be: NccOU 6/8 PH 6/8. "Dynamic<unk> " defines the ability of the eye to<unk> ly discern fine detail in a moving object.
Visual acuity
Color vision
Vernier acuity
Visual field
Vision
Intraocular lens power calculation
Monocular vision
Neurological pupil index