Difference between revisions of Astigmatism
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==Understanding astigmatism== | ==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. ‘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]] | [[File:Astigmatism.svg|Astigmatism]] | ||
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===Stenopaeic slit=== | ===Stenopaeic slit=== | ||
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 | 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 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. | It may be easier to visualize the effect by comparing it to [[Chromatic Aberration|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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* If the blue light is focused correctly, the green and red will be focused beyond the retina and will be 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 | 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 | 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, 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. | 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. | ||
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The average person with medium-to-high myopia should wait until having reduced norms/diffs twice each before reducing cylinder for the first time. Then wait a minimum of 8 weeks before introducing the next spherical reduction, and wait 6 months before introducing the next cylinder reduction. | The average person with medium-to-high myopia should wait until having reduced norms/diffs twice each before reducing cylinder for the first time. Then wait a minimum of 8 weeks before introducing the next spherical reduction, and wait 6 months before introducing the next cylinder reduction. | ||
Regardless of how the myope reduces their lenses, the goal of each reduction is to have a small amount of | 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=== | ===Axis=== | ||
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==Spherical equivalent== | ==Spherical equivalent== | ||
{{Main|Diopters#Spherical Equivalent}} | {{Main|Diopters#Spherical Equivalent}} | ||
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 | 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”. | ||
A common [[reduction]] is to trade 0.5 cylinder to its spherical equivalent of 0.25 sphere (of the same sign). | A common [[reduction]] is to trade 0.5 cylinder to its spherical equivalent of 0.25 sphere (of the same sign). | ||