Difference between revisions of Astigmatism

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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===

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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.

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,

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 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===
@@ Line 25: / Line 29: @@
 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,