Cylindrical lenses were not used to adjust image parameters or to correct astigmatism in the past. Research studies in the early 70ss showed that a visual impairment called astigmatism could be corrected using cylindrical lenses. Astigmatism occurs when the cornea is not completely round. This condition causes blurred vision under certain angles. For this reason, when astigmatism is measured, the spherical radius (sph) is matched with the cylindrical radius (cyl) and the lens axis (axis).
There are two methods to establish diopter values to correct astigmatism: by either the positive or the negative cylindrical radius. The negative (minus) values are more often applied in ophthalmology. On the contrary, the optics prefer the positive (plus) values. Both approaches are correct even if it might not seem so.
Values applied to correct astigmatism if the cylindrical radius is negative:
sph -1.00 dpt cyl -2.00 dpt axis 30°
The diopter values in the discussed example can be also expressed as follows:
sph -3.00 dpt cyl +2.00 dpt axis 120°
The diopter count for our eyes is expressed differently, while our eyesight has not changed. Obtain your diopter count yourself calculated as below.
- Add the diopter value and the cylindrical radius (sph + cyl) to obtain the amount of lens power called a sphere (sph).
- Do not convert the cylindrical radius, simply change the minus sign to the plus sign and vice versa.
- Shift the cylindrical axis by adding 90°.
The prescribed correction seems to have significantly changed, which is not the case. Do not worry and don’t get confused. We are speaking about the same correction; it is simply expressed as either a positive (+) or a negative (-) cylinder. Let’s remember shoe sizes, shall we? Do you prefer size 41 or size 7?