Difference between revisions of Astigmatism

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'''Astigmatism''' is a really annoying 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 ~~no astigmatism, and need no extra lens~~ power, or the one where you have the most ~~and need the most extra lens power~~). Astigmatism is compensated with [[cylinder]] lenses. A cylinder lens adds power along one particular meridian of the eye.

'''Astigmatism''' is a really annoying 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.

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==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. It is often described as a 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:

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 a 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. ~~(This is a similar sort of~~ effect to [[Chromatic Aberration]], ~~where~~ different ~~wavelengths~~ are focused ~~at different distances~~.)

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.

===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.

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.

* The required "spherical equivalence" is the average of corrections required to bring blue or red 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.

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-'''Astigmatism''' is a really annoying 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 no astigmatism, and need no extra lens power, or the one where you have the most and need the most extra lens power). Astigmatism is compensated with [[cylinder]] lenses. A cylinder lens adds power along one particular meridian of the eye.
+'''Astigmatism''' is a really annoying 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.
@@ Line 5: / Line 5: @@
 ==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. It is often described as a 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:
+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 a 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.  (This is a similar sort of effect to [[Chromatic Aberration]], where different wavelengths are focused at different distances.)
+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.
+===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.
+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.
+* The required "spherical equivalence" is the average of corrections required to bring blue or red 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.

Difference between revisions of Astigmatism

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