Understanding the intricacies of eyeglass prescriptions can be daunting, especially when dealing with cylinder power, which corrects astigmatism. Often, you'll see prescriptions written with either a plus (+) or minus (-) cylinder. While both represent the same correction, the difference lies in the axis. This guide will help you navigate the conversion between plus and minus cylinder, explaining the process and dispelling any confusion.
What is Cylinder Power in an Eyeglass Prescription?
Cylinder power, denoted by "CYL" or "Cyl," in your eyeglass prescription, indicates the amount of correction needed to address astigmatism. Astigmatism is a refractive error where the cornea (the front surface of your eye) is not perfectly spherical, causing blurry or distorted vision. A plus cylinder (+) indicates a refractive error where the cornea is steeper in one meridian than another, while a minus cylinder (-) indicates a flatter area.
How to Convert Plus Cylinder to Minus Cylinder (and vice versa)
The conversion process isn't simply changing the sign. It involves a crucial step involving the axis. The key to converting between plus and minus cylinder is understanding that they represent the same correction, just expressed differently.
The Conversion Formula:
To convert from plus cylinder to minus cylinder (or vice versa):
- Change the sign: Change the "+" to a "-" or vice versa.
- Adjust the axis: Add 90 degrees to the original axis. If the sum exceeds 180 degrees, subtract 180 degrees.
Example:
Let's say you have a prescription with +1.00 CYL x 10°. To convert this to minus cylinder:
- Change the sign: +1.00 becomes -1.00.
- Adjust the axis: 10° + 90° = 100°.
Therefore, +1.00 x 10° is equivalent to -1.00 x 100°.
Another Example (Axis over 180°):
Let's consider a prescription with +2.00 CYL x 150°. To convert:
- Change the sign: +2.00 becomes -2.00.
- Adjust the axis: 150° + 90° = 240°. Since this is over 180°, subtract 180°: 240° - 180° = 60°.
Therefore, +2.00 x 150° is equivalent to -2.00 x 60°.
Why are there two ways to express cylinder power?
Different optometrists and ophthalmologists may use different notations based on their preference or the type of equipment used during the eye exam. Both notations represent the same correction, providing the same visual outcome. The choice is largely a matter of convention and doesn't affect the accuracy of the prescription.
Does the sphere power change during conversion?
No, the sphere power (SPH) remains unchanged during the conversion process. Only the cylinder power and axis are altered.
Can I convert my prescription myself?
While you can perform the mathematical conversion as described above, it's crucial to consult your eye doctor or optometrist before making any changes to your prescription. They possess the expertise to interpret your prescription and ensure the accuracy and safety of your eyeglass lenses. Self-adjusting your prescription can be detrimental to your eyesight.
What if my prescription doesn't have a cylinder?
If your prescription doesn't have a cylinder power (CYL 0.00), it means you don't have astigmatism and this conversion isn't applicable.
This guide provides a clear understanding of how to convert between plus and minus cylinder power in an eyeglass prescription. However, always remember that your eye care professional is the best source of information regarding your specific vision needs. Never attempt to adjust your prescription without professional guidance.
