Difference between revisions of Astigmatism

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'''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''' 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 or the most focusing power). 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.
Astigmatism often reduces spontaneously as myopia is corrected.
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[[File:Astigmatism.svg|Astigmatism]]
[[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.
Most lens diagrams show only a single vertical plane. In the real world, of course, there is a full cone of light arriving at the lens from the source object. This diagram shows two different cross-sections through the incident cone of light, aligned with the two axes. The 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, resulting in a smeared image along the axis.


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


===Analogy with Chromatic Aberration===
===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.
It may be easier to visualize the effect by comparing it to [[Chromatic Aberration]]. In both cases, an extra variable means that different parts of the light are focused differently.


[[File:Chromatic aberration lens diagram.svg|Chromatic aberration lens diagram]]
[[File:Chromatic aberration lens diagram.svg|Chromatic aberration lens diagram]]
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(Unfortunately the astigmatism diagram choose red and blue the wrong way round !)
(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 red light is focused on the retina, the green/blue light is focused in front of it and is blurred.
* If the blue light is focused correctly, green and red are focused beyond the retina and is blurred.
* If the blue light is focused correctly, the green and red will be focused beyond the retina and will be blurred.
* It's not possible to get everything into focus using only spherical lenses.
* 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.
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.
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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.
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.
The analogue of the Stenopaeic slit is this model, which is a simple colour filter : by allowing only monochromatic light to enter the eye, the spherical correction can be measured separately for each colour.


If the object is moved away, beyond your blur horizon, so that your eye can no longer keep the green light focused,
If the object is moved away, beyond your blur horizon, so that your eye can no longer keep the green light focused,
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in astigmatism. Adding some spherical correction would allow you to push the green back into focus.
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...)
Chromatic aberration could be treated by adding some material that 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 bit of a stretch...)


==Irregular Astigmatism==
==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.
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 also rotated, resulting in multiple (blurred) images being perceived on the retina.


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