Astigmatism

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 often reduces spontaneously as myopia is corrected.

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.

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.

The spherical equivalent of cylinder lenses may be useful, to simplify the reduced lens path. 0.50 cylinder is equivalent to 0.25 sphere. 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 perfectly tailored for the astigmatic blur, so it leaves a small amount of "useful blur".

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.

Resources

• EndMyopia Blog - The Definitive Guide: What Is Astigmatism
• EndMyopia Blog - all astigmatism articles