Difference between revisions of Astigmatism
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(→Understanding astigmatism: A first stab at explaining the optics, with pictures, though I could be hopelessly wrong on all this) |
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[[File:Astigmatism (Eye).png|Astigmatism (Eye)]] | [[File:Astigmatism (Eye).png|Astigmatism (Eye)]] | ||
==Childhood Astigmatism== | |||
Astigmatism in young children often changes after they reach school age.<ref>{{Cite journal |last=Dobson |first=V. |last2=Fulton |first2=A. B. |last3=Sebris |first3=S. L. |date=1984-01-01 |title=Cycloplegic refractions of infants and young children: the axis of astigmatism. |url=https://iovs.arvojournals.org/article.aspx?articleid=2159731 |journal=Investigative Ophthalmology & Visual Science |language=en |volume=25 |issue=1 |pages=83–87 |issn=1552-5783}}</ref> | |||
==Reducing astigmatism== | ==Reducing astigmatism== | ||
As with myopia, astigmatism should be tackled in small steps when selecting lenses for [[differential]] or [[normalized]] glasses. If only a small amount of cylinder correction is present, say 0.25 [[diopters]], the cylinder correction can be dropped, with no other changes. Otherwise, cylinder should be reduced in small increments. If sphere is being reduced, cylinder should not be changed at the same time, and conversely, if cylinder is being reduced then sphere should not be changed. The only time that both sphere and cylinder should be changed, is when converting to the spherical equivalent. | As with myopia, astigmatism should be tackled in small steps when selecting lenses for [[differential]] or [[normalized]] glasses. If only a small amount of cylinder correction is present, say 0.25 [[diopters]], the cylinder correction can be dropped, with no other changes. Otherwise, cylinder should be reduced in small increments. If sphere is being reduced, cylinder should not be changed at the same time, and conversely, if cylinder is being reduced then sphere should not be changed. The only time that both sphere and cylinder should be changed, is when converting to the spherical equivalent. | ||
The link between astigmatism strength and visual acuity is weak. How your eyes and [[visual cortex]] respond to astigmatism is a greater factor in visual acuity than the number of dipoters.<ref>{{Cite journal |last=Remón |first=Laura |last2=Tornel |first2=Marta |last3=Furlan |first3=Walter D. |date=2006-05 |title=Visual Acuity in Simple Myopic Astigmatism: Influence of Cylinder Axis |url=https://journals.lww.com/optvissci/Abstract/2006/05000/Visual_Acuity_in_Simple_Myopic_Astigmatism_.11.aspx |journal=Optometry and Vision Science |language=en-US |volume=83 |issue=5 |pages=311–315 |doi=10.1097/01.opx.0000216099.29968.36 |issn=1538-9235}}</ref> If the standard advice for correcting astigmatism does not work for you, you may need a more gradual reduction. | |||
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". The resulting spherical equivalent is not intended to compensate for the asymmetry of the lens, so it will introduce some directional blur. | 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". The resulting spherical equivalent is not intended to compensate for the asymmetry of the lens, so it will introduce some directional blur. | ||