Snellen chart

2026-10-15T05:56:02Z

Endmyopiauser: Link to the free online eye test (10-15)

{{Infobox diagnostic
| name = Snellen chart
| image = Snellen chart.svg
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| purpose = Snellen chart is used to estimate visual acuity
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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, most myopes can be corrected to the [[20/20]] line unless they have visual acuity problems other than myopia. There is no direct correlation between visual acuity and [[myopia]]. The Snellen chart is the most commonly used way of testing if someone has sufficient corrected visual acuity to drive legally. Your [[optometrist]] will use the Snellen chart as a reference, to see what refraction will allow you to read the lowest on the chart.

You can check your own eyesight at home with the free [https://start.endmyopia.org/tools/online-eye-test online eye test], which sizes a real letter chart to your screen and your distance.
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==Definition of the numbers==

The acuity 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
* 20/16 means 4 arc-minutes at 20 feet
* 6/12 means 10 arc-minutes at 6m

The critical gaps that need to be resolved are 1 arc-minute, so that's really what defines the required acuity.

==How to use a Snellen Chart to measure visual acuity==
# Read the documentation that came with your chart to determine what distance it is designed to be viewed at. This is usually 10 feet, 20 feet, 3 m or 6m. It may say right on the chart, on the back, or in the description of the product where you got it.
# Post the chart at eye level
# Make a mark on the floor at the distance the chart is rated for.
# Stand so that your face is above that line.
# Gently cover one eye (without actually touching it).
# Read the lowest line you can see clearly.
# Attempt to read the next line down.
# Have an assistant tell you if you got the letters right or move forward to check. If you got more than half of the letters right on that line, then the ratio marked for that line is your visual acuity. Record.
# Repeat steps 5-8 with the other eye covered.
# Repeat once more with both eyes together.

Please note: Unless you are in low myopia territory snellen measurements are intended to measure your vision with your distance correction.

Also important to know: Lighting will effect your results and due to the nature of the diopter bubble longer charts (6m/20ft) are more accurate than the short charts (3m/10ft), though both are perfectly acceptable methods to track vision trends over time.

==How to use a Snellen Chart with refraction==
{{main|Trial lens kit}}

==Tools==
* [http://www.i-see.org/block_letter_eye_chart.pdf Block Letter Eye Chart with calibration measurement instructions 20feet/6meters)]
* [http://www.i-see.org/etdrs/etdrs-1.pdf ETDRS Snellen version 1 20feet/6meters]
* [http://www.i-see.org/etdrs/etdrs-2.pdf ETDRS Snellen version 2 20feet/6meters]

* [http://www.i-see.org/etdrs/etdrs-r.pdf ETDRS Snellen version 3 20feet/6meters]

* [https://www.provisu.ch/images/PDF/Snellenchart_en.pdf Printable Snellen Chart 2.8m(9 feet)]
* [https://www.teachengineering.org/content/cub_/activities/cub_human/cub_human_lesson06_activity1_eyechart.pdf Printable Snellen Eye Chart 6m(20 feet)]
* [https://github.com/mminer237/denim-weasel Snellen chart generator]
* [https://eyes.arizona.edu/sites/default/files/eyechart.pdf Banner Eye Care Visual Acuity Test (BECVAT) V 2.0]
* Some people with astigmatism may use the [https://en.wikipedia.org/wiki/Stenopaeic_slit stenopaeic slit]<ref>https://community.endmyopia.org/t/francois-journey-to-20-20/10428/3</ref> as a measuring tool
* Alternatively, you can use an [http://www.i-see.org/astigmirror.pdf astigmatism mirror/fan] along with [https://endmyopia.org/diy-tools-how-to-measure-your-astigmatism-diopters/ this guide for estimating astigmatism].

==See Also==

* [[Astigmatism_measurement]]
* [[Guide:How to measure your eyesight]]

==References==
{{reflist}}

[[Category:Articles]]

Defocus method

2026-08-10T07:39:43Z

Endmyopiauser: Create Defocus method: mechanism, lineage from Reduced Lens Method, sourced to endmyopia.org

The 'Defocus Method' is the current name for the [[EndMyopia]] approach to reducing myopia. It is the evolution of the 'Reduced Lens Method', first published on endmyopia.org on 29 November 2012. The rename followed the causality: the early version was described in terms of the tool, reduced lenses, while the method is now named for the mechanism those lenses control, which is defocus.

== Defocus ==

Defocus describes where light comes to a point relative to the retina.

* 'Myopic defocus': light focuses in front of the retina.
* 'Hyperopic defocus': light focuses behind the retina.

Both are blur, and they can look alike, but the retina distinguishes them and responds in opposite directions.

== Mechanism ==

Eyes grow into focus through emmetropization, guided by visual feedback. The sign of defocus is the signal:

* Sustained 'hyperopic defocus' drives axial elongation. This is the standard laboratory method for inducing myopia in animal models, known as lens induced myopia.
* Sustained 'myopic defocus' slows or halts elongation.

The response is largely local to the retina and persists when the optic nerve is severed, so it is not mediated by the brain. The peripheral retina matters a great deal.

== Applied ==

A full distance correction worn for close work produces accommodative lag, leaving the image slightly behind the retina, which is hyperopic defocus delivered for hours a day. Standard single vision lenses also leave the periphery focused behind the retina.

The Defocus Method reverses the sign of that input:

* [[Differentials]] remove the hyperopic defocus load during close work.
* [[Normalized]] keeps a small myopic defocus present at distance, the [[blur horizon]].
* [[Active Focus]] engages that edge.

Correction is reduced further as measurements improve, at roughly one diopter per year.

== Industry parallel ==

Every mainstream myopia control product works by imposing peripheral myopic defocus, including DIMS lenses (Hoya MiyoSmart), dual focus contact lenses such as MiSight, Zeiss MyoCare, and orthokeratology. The mechanism is not disputed. It is the basis of the myopia control category.

== Origin ==

The term and the method originate with Jake Steiner. Its component terms were coined on endmyopia.org between September 2012 and July 2013, with dated first published use recorded in the [https://endmyopia.org/endmyopia-glossary/ Endmyopia glossary]. Full biology: [https://endmyopia.org/defocus-method/ The Defocus Method].

Endmyopia and Active Focus are registered trademarks of Jake Steiner.

Endmyopia Wiki - User contributions [en]

Snellen chart

Defocus method