Difference between revisions of Snellen chart
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→Definition of the numbers: typo
(move definitions from the 20/20 page) |
m (→Definition of the numbers: typo) |
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==Definition of the numbers== | ==Definition of the numbers== | ||
The actuity is expressed as the | The actuity is expressed as the ratio 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 | * 20/20 means 5 arc-minutes at 20 feet | ||
* 20/16 means 4 arc-minutes at 20 feet | * 20/16 means 4 arc-minutes at 20 feet |