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A '''Snellen chart''' is a standard method of measuring [[visual acuity]]. A chart is rated for the distance it should be viewed at, and the lowest line that can be read has a visual acuity result number next to it. The standard distance for Snellen charts is 20 feet or 6 meters. Smaller versions are available for use in smaller indoor spaces. Generally, the [[20/20]] line is the baseline to which your optometrist will aim for your vision correction. The Snellen chart is the most commonly used way of testing if someone has sufficient corrected visual acuity to drive legally. There is no direct correlation between visual acuity and [[myopia]]. Your [[optometrist]] will use the Snellen chart as a reference, to see what refraction will allow you to read the lowest on the chart. | A '''Snellen chart''' is a standard method of measuring [[visual acuity]]. A chart is rated for the distance it should be viewed at, and the lowest line that can be read has a visual acuity result number next to it. The standard distance for Snellen charts is 20 feet or 6 meters. Smaller versions are available for use in smaller indoor spaces. Generally, the [[20/20]] line is the baseline to which your optometrist will aim for your vision correction. The Snellen chart is the most commonly used way of testing if someone has sufficient corrected visual acuity to drive legally. There is no direct correlation between visual acuity and [[myopia]]. Your [[optometrist]] will use the Snellen chart as a reference, to see what refraction will allow you to read the lowest on the chart. | ||
==Definition of the numbers== | |||
The actuity is expressed as the ration of the distance (usually 20ft or 6m but can be less) to the letter size, expressed as the distance at which the letter subtends an angle of 5 arc-minutes. (Or equivalently, as multiples of the height at which the letter subtends 5 arc-minutes at a distance of 1 unit of distance). 5 arc-minutes just means 5/60 of a degree. So | |||
* 20/20 means 5 arc-minutes at 20 feet | |||
* 20/16 means 4 arc-minutes at 20 feet | |||
* 6/12 means 10 arc-minutes at 6m | |||
==How to use a Snellen Chart to measure visual acuity== | ==How to use a Snellen Chart to measure visual acuity== | ||