Visual acuity refers to the angular subtense of the smallest size of detail that the eye can resolve. It is usually expressed as a fraction where the numerator refers to the testing distance in meters (feet in the United States). Six meters is the reference distance (20 feet in the United States), and the denominator changes proportionally according to the change in size of the lettering, while reading from the same reference distance. The easiest way to interpret a visual acuity is to divide out the fraction. For example, average visual acuity is 6/6 (in feet, 20/20), which divides out to 1.0, or 100%. Although this is the theoretical normal level of visual acuity, most children do better than this, typically achieving 6/4 (20/30), which divides out at 1.5 or 150%. The bottom line of many letter charts is 6/3 (half the size of 6/6 letters), and the top line of most letter charts is 6/60. A 6/60 letter is 10 times the size of a 6/6 letter and 6/60 divides out at 0.1 or 10% of "normal" visual acuity.
Most of the near print that 6- or 7-year-old children would be expected to read is equivalent to about 6/24, and typical text for a 9-year-old is equivalent to about 6/18. The smallest print that any children are expected to read (e.g., dictionaries) is equivalent to about 6/12. Although this is approximately twice the threshold acuity, this margin may be necessary for comfortable vision.
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