The precision of vision
Acuity is the measure of your ability to tell fine detail and visual acuity is a measure of your ability to tell fine visual detail, i.e., the ability to distinguish details and shapes of objects. This is best when the image falls on the fovea of the eye and is therefore a measure of your central vision (vision at and about the fovea).
Tests for the acuteness of central vision are common in routine medical and ophthalmological examinations. Provided that the optical system of the eye forms distinct images of the objects viewed (which is not the case in many uncorrected errors of refraction), visual acuity depends on the resolving power of the retina. With foveal vision two points can be distinguished as two if they subtend at the nodal point of the eye a visual angle of one minute.
Snellen's Test Types are the most commonly used tests for acuity of distant vision and you may have encountered them when going for a learner's permit for driving or when you went to the doctor. The letters on these charts are of various sizes; if seen from the distances stated for each line, they subtend a visual angle of five minutes, the relevant details of the letters then subtending visual angles of one minute. The largest letter has the appropriate size for a distance of 60 m. There are rows of letters for 36, 24, 18, 12, 9, 6, 5 and 4 m distance.
For near vision acuity, the usual test types are those by Jaeger and these are now common among optometrists – hence you may have encountered them when having your eyes checked at such an establishment. It is a card on which a text is printed, with the various paragraphs of the text having sizes of type increasing from 0.37mm (Jaeger no. 1) to 2.5mm, with successive numbers indicating coarser types. The principle is the same as for the Snellen's chart. The card is to be held at a fixed distance from the eye and the person determines the smallest row of text that they can read. This determines their visual acuity.
Testing your visual acuity
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This test has been embedded from https://web.archive.org/web/20170131174245/http://www.smbs.buffalo.edu/oph/ped/IVAC/IVAC.html