Difference between revisions of 20/20 correction
No edit summary |
(tighten up on defns of acuity) |
||
| Line 1: | Line 1: | ||
'''''20/20 correction''''' is a correction you normally get from a licensed optician, which corrects your eyesight up to a degree of [[visual acuity]] that an [[emmetropic]] (or "normal-seeing") person achieves on a [[Snellen Chart]]. | '''''20/20 correction''''' is a correction you normally get from a licensed optician, which corrects your eyesight up to a degree of [[visual acuity]] that an [[emmetropic]] (or "normal-seeing") person achieves on a [[Snellen Chart]]. The visual acuity is defined as the distance (usually 20ft or 6m but can be less), divided by the letter size, expressed as the distance at which the letter subtends an angle of 5 arc-minutes. (Or as multiples of the height at which the letter subtends 5 arc-minutes at a distance of 1.) (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 | |||
If you can see 20/15 or even better, this means that your are [[overcorrected]]. (It is not necessarily considered super-human actuity, but it is unnecessary to correct to this level.) | |||
It is very difficult to perform [[Active Focus]] at this strength of glasses, as there is not enough of a [[blur horizon]] to produce [[stimulus]] - see [[Distance vision]]. | It is very difficult to perform [[Active Focus]] at this strength of glasses, as there is not enough of a [[blur horizon]] to produce [[stimulus]] - see [[Distance vision]]. | ||
Revision as of 20:08, 15 June 2020
20/20 correction is a correction you normally get from a licensed optician, which corrects your eyesight up to a degree of visual acuity that an emmetropic (or "normal-seeing") person achieves on a Snellen Chart. The visual acuity is defined as the distance (usually 20ft or 6m but can be less), divided by the letter size, expressed as the distance at which the letter subtends an angle of 5 arc-minutes. (Or as multiples of the height at which the letter subtends 5 arc-minutes at a distance of 1.) (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
If you can see 20/15 or even better, this means that your are overcorrected. (It is not necessarily considered super-human actuity, but it is unnecessary to correct to this level.)
It is very difficult to perform Active Focus at this strength of glasses, as there is not enough of a blur horizon to produce stimulus - see Distance vision.