Astigmatism

Astigmatism means that the cornea is not evenly curved like a round ball (Fig. 1), but with different curvatures (radii) is shaped like an egg (Fig. 2).

spherical form with equal radii – no astigmatism

Figure 1: spherical form with equal radii – no astigmatism

Egg-shape with different curvatures (radii) – astigmatism

Figure 2: egg-shape with different curvatures (radii) – astigmatism

The term “astigmatism” is derived from the Greek word “stigma = point”; the word means “without a point”. A point is not displayed as a point on the retina (fundus) but, with different curvatures of the cornea, as a small bar.

In one direction, there is a shallow curvature; at 90° opposite is a steep curvature of the cornea. This results in different strengths in the two directions, which can be compensated with toric lenses. For this purpose, the optician/ optometrist measures in which axis and how strong the flattest part of the cornea is and how strong the steepest part.

This will result in two values (Ex. 2); it can be associated with both myopia (Ex. 1) as well as both hyperopia. Yet it may also be that with a value for myopia, the other value is a correct farsightedness (Ex. 3).

Example 1:
1. value sph -1.25 dpt; 90°      2. value sph -1.75 dpt; 180°

Example 2:
1. value sph +1.75 dpt; 110°  2. value sph +1.50 dpt; 20°

Example 3:
1. value sph +0.50 dpt; 35°   2. value sph -1.00 dpt; 125°

The prescription for your glasses could look like this:

Example 1:
sph -1.25 dpt cyl -0.50 dpt Ax 90°    or   sph -1.75 dpt cyl +0.50 dpt Ax 180°

Example 2:
sph +1.75 dpt cyl -0.25 dpt Ax 11   or   sph +1.50 dpt cyl +0.25 dpt Ax 20°

Example 3:
sph +0.50 dpt cyl -1.50 dpt Ax 35°     or   sph -1.00 dpt cyl +1.50 dpt Ax 125°

It is, then, always a combination of the two measured values specified (sphero-cylindric combination): In the first example, the first value is first recorded as a sphere (sph -1.25 diopters). Then the value is calculated, which is required in order to arrive from the first to the second value ((-1.25) -0.50 = -1.75). This value is the cylindrical value (cyl -0.50 diopters). The axis applies to the spherical value (axis 90°). This is the “minus cylinder” notation.

It can also be noted the other way round: the second value for a sphere (sph -1.75 diopters). Then the value is calculated, required in order from the second to come to the first value (-1.75) + 0.50 = -1.25). This value is the cylindrical value (cyl +0.50 diopters). The axis applies to the spherical value (axis 180°). This is the “plus cylinder” notation.

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