Difference between revisions of Astigmatism

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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.

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:

[[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.)

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)]]

==Reducing astigmatism==

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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.
+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:
+[[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.)
+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)]]
 ==Reducing astigmatism==

Difference between revisions of Astigmatism

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