Astigmatism

2021-02-19T17:11:08Z

Idob: /* Understanding astigmatism */

'''Astigmatism''' is an eye condition that means you have blur in a specific direction, or [[axis]] (technically, depending on the notation used for your prescription, the axis may indicate the angle of the eye's meridian where you have the least focusing power, or the one where you have the most). Astigmatism is compensated with [[cylinder]] lenses. A cylinder lens adds power along one particular meridian of the eye.

Astigmatism often reduces spontaneously as myopia is corrected.

==Understanding astigmatism==

Astigmatism is caused by an irregularly shaped cornea or lens. The first is called corneal astigmatism, which is the more common form, and the second is called lenticular astigmatism. "Regular" astigmatism is often described as having the cornea shaped like a rugby ball, rather than spherical like a basketball. The optics of an ''idealised'' lens of such a form would cause incoming light on different planes (corresponding to the two principal axes of the lens) to be focused at different offsets beyond the lens.

[[File:Astigmatism.svg|Astigmatism]]

Most diagrams of lenses show only a single vertical plane. In the real world, of course, there is a full cone of light arriving on the lens from the source object. This diagram shows two different cross-sections through the incident cone of light, aligned with the two axes. Rays in the horizontal cross-section (which contains the stronger curve) are focused earlier - at the label 'T' (for 'Tangential'). The rays in the vertical cross-section (the 'Saggital') are focused further behind, at 'S'. The other rays around the light cone are focused at points in between the two, giving an image smeared out along the axis.

===Stenopaeic slit===

The Stenopaeic slit is a simple tool which can be used in diagnosis / measurement of astigmatism. It is simply a disk with a narrow slit which can be rotated to find the clearest image. By reducing light coming in the "wrong" direction, the spherical correction on each axis can be measured directly.

===Analogy with Chromatic Aberration===

It may be simpler to picture the effect by comparing with [[Chromatic Aberration]]. In both cases, an extra variable means different parts of the light are focused differently.

[[File:Chromatic aberration lens diagram.svg|Chromatic aberration lens diagram]]

(Unfortunately the astigmatism diagram choose red and blue the wrong way round !)

* If the red light is focused on the retina, the green/blue light is focused in front, and is blurred.
* If the blue light is focused correctly, green and red are focused beyond the retina and is blurred.
* It's not possible to get everything into focus using only spherical lenses.
As a compromise, focusing the green light on the retina causes a little bit of myopic blue blur and hyperopic red blur. This corresponds to the "circle of least confusion" in astigmatism.

On a prescription, there are two different conventions for specifying the cylinder. This corresponds to either quoting the spherical correction to focus red on the retina, and then the additional "minus" required to focus blue; or a spherical correction for blue, and then how much that can be reduced by for red. The average of the two, or the "spherical equivalence", is then the correction required to put green on the retina.

The analogue of the Stenopaeic slit is this model is a simple coloured filter : by allowing only monochromatic light into the eye, the spherical correction for each colour can be measured separately.

If the object is moved away, beyond your blur horizon, so that your eye can no longer keep the green light focused,
all colours will suffer myopic blur, but blue will have the worst blur. This corresponds to the directional blur
in astigmatism. Adding some spherical correction would allow you to push the green back into focus.

Chromatic aberration could be treated by adding some material which applies the opposite chromatic error - bending the blue light out a bit more than the red light, to cancel the error introduced by the eye. This corresponds to cylinder correction. (But as with all analogies, it's starting to stretch a bit thin...)

==Irregular Astigmatism==

A real cornea, of course, doesn't conform to expectations. Being messier, it just has a bulge, which means that the image is not only smeared out along the axis, but is rotated, resulting in multiple (blurred) images being perceived on the retina.

[[File:Astigmatism (Eye).png|Astigmatism (Eye)]]

==Childhood Astigmatism==
Astigmatism in young children often changes after they reach school age.<ref>{{Cite journal |last=Dobson |first=V. |last2=Fulton |first2=A. B. |last3=Sebris |first3=S. L. |date=1984-01-01 |title=Cycloplegic refractions of infants and young children: the axis of astigmatism. |url=https://iovs.arvojournals.org/article.aspx?articleid=2159731 |journal=Investigative Ophthalmology & Visual Science |language=en |volume=25 |issue=1 |pages=83–87 |issn=1552-5783}}</ref>

==Reducing astigmatism==
As with myopia, astigmatism should be tackled in small steps when selecting lenses for [[differential]] or [[normalized]] glasses. If only a small amount of cylinder correction is present, say 0.25 [[diopters]], the cylinder correction can be dropped, with no other changes. Otherwise, cylinder should be reduced in small increments. If sphere is being reduced, cylinder should not be changed at the same time, and conversely, if cylinder is being reduced then sphere should not be changed. The only time that both sphere and cylinder should be changed, is when converting to the spherical equivalent.

The link between astigmatism strength and visual acuity is weak. How your eyes and [[visual cortex]] respond to astigmatism is a greater factor in visual acuity than the number of dipoters.<ref>{{Cite journal |last=Remón |first=Laura |last2=Tornel |first2=Marta |last3=Furlan |first3=Walter D. |date=2006-05 |title=Visual Acuity in Simple Myopic Astigmatism: Influence of Cylinder Axis |url=https://journals.lww.com/optvissci/Abstract/2006/05000/Visual_Acuity_in_Simple_Myopic_Astigmatism_.11.aspx |journal=Optometry and Vision Science |language=en-US |volume=83 |issue=5 |pages=311–315 |doi=10.1097/01.opx.0000216099.29968.36 |issn=1538-9235}}</ref> If the standard advice for correcting astigmatism does not work for you, you may need a more gradual reduction.

The '''spherical equivalent''' of cylinder lenses may be useful, to simplify the reduced lens path. 0.50 cylinder means the power varies from 0D on one axis to 0.5D on the perpendicular axis. This can be substituted by the spherical power with the averaged value of 0.25D. For example, a full prescription of "-1.00 Sphere -1.50 Cylinder" could be converted to "-1.75 Sphere". The resulting spherical equivalent is not intended to compensate for the asymmetry of the lens, so it will introduce some directional blur.

Regardless of how the myope reduces their lenses, the goal of each reduction is to have a small amount of "useful blur", to be cleared up with [[active focus]] and good habits.
===Axis===
{{Main|Axis}}
When reducing cylinder value, you should never change the axis value if it has been consistent for some time. The axis only refers to the orientation in which the cylinder is affecting your eyes, and any reduction of cylinder will happen on this orientation.

==Resources==
* [https://endmyopia.org/the-definitive-guide-astigmatism/ EndMyopia Blog - The Definitive Guide: What Is Astigmatism]
* [https://endmyopia.org/tag/astigmatism-2/ EndMyopia Blog - all astigmatism articles]
* [https://endmyopia.org/diy-tools-how-to-measure-your-astigmatism-diopters/ How To Measure Your Astigmatism Diopters]
* [https://visiontools.netlify.app/ Vision tool]
* [https://community.endmyopia.org/t/having-trouble-figuring-out-my-astigmatism/4843/6 Astigmatism Assasin's guide]
* [https://www.youtube.com/watch?v=KDgU4AoFsA4 Fixing Astigmatism - Jake Steiner]

==References==
{{reflist}}

[[Category:Articles]]
[[Category:Eye conditions]]

Guide:Reducing differentials

2021-02-19T16:56:32Z

Idob: /* Complicated way with math */

{{shortcut|Guide:RedDiffs}}
This guide will show you how to reduce your differential prescription. Can't have any of that nasty [[close-up]] without the reduced lenses.

If you don't use differentials at all, or are overcorrected for a given distance, your progress will be stunted. Accommodation of the eye to [[hyperopic defocus]] is the main reason [[axial elongation]] happens in the first place.
==Simple way==
The EndMyopia process will probably involve a lot of trial and error. Bearing this in mind, Jake recommends you reduce from your glasses prescription by about '''1.25 diopters'''.<ref name=ChooseReduction>{{cite jake|https://endmyopia.org/choose-reduced-glasses-prescription-differential-close-friendly-glasses/|How To Choose A Reduced Glasses Prescription (Differential / Close-Up Friendly Glasses)}}</ref> This will depend on the distance you work from the screen, so reduce it by less if you work farther from the screen. Between 1-2 diopters is a good range - anything more than that and you'll probably want to double check it's a good idea.

==Complicated way with math==
The base point is an [[emmetropic eye]] with 20/20 vision.

Everyone has a different level of myopia, and depending on this you can calculate the required amount of correction to just see the object, based on how far away it is.

Firstly, find out the distance from you to the object. In plain English, how far away is the object in centimetres when (without glasses) you can still just see it in focus?

Type in the cm value you get into [https://endmyopia.org/focal-calculator/calc.html EndMyopia diopter calculator] to figure out the '''difference''' between your full strength prescription glasses, and what you actually need to see the screen.

Then, take the difference and subtract it from your full strength prescription glasses. ''Voila!'' you have a differential prescription.

===Example===
Suppose you have [[Guide:OD/OS|OD/OS]] -4.25/-3.25, and you see, or want to see your monitor 60cm away from where you sit.

Go to the [https://endmyopia.org/focal-calculator/calc.html calculator], and enter in 60cm. The result is '''-1.75'''. Subtract this value from both of your eyes (be careful with negative numbers).

<math>OD\ (right\ eye): -4.25-(-1.75)=-2.5</math>

<math>OS\ (left\ eye): -3.25-(-1.75)=-1.5</math>

So, you should use OD/OS -2.5/-1.5, if 60cm is the main distance you perform [[close-up activity]]. Simple!

==References==
{{reflist}}

== See also ==
[[Guide:Reducing normalized]]

[[Category:Guides]]

Endmyopia Wiki - User contributions [en]

Astigmatism

Guide:Reducing differentials