New Sphere (S’):
New Cylinder (C’):
New Axis (A’):
The Plus to Minus Cylinder Calculator is a specialized tool designed for optometrists, ophthalmologists, and eyewear professionals. It converts eyeglass prescriptions from the “plus cylinder” form to the “minus cylinder” form. This conversion is critical for accurately crafting lenses that correct for astigmatism—a common condition where the eye does not focus light evenly onto the retina, causing blurred vision.
Formula of Plus to Minus Cylinder Calculator
The core of this calculator lies in its formula:
- NewSphere (S’): S’ = S + C
- NewCylinder (C’): C’ = -C
- NewAxis (A’): A’ = A + 90°
Here, S represents the original sphere value, indicating the degree of nearsightedness or farsightedness. C is the original cylinder value, which corrects for astigmatism and should be positive in a plus cylinder prescription. A is the original axis value, denoting the lens meridian that does not contain cylinder power, measured in degrees between 0 and 180.
The axis adjustment is crucial, involving an addition of 90 degrees to the original axis. If this calculation results in a value over 180 degrees, we subtract 180 to keep the axis within the standard range of 1 to 180 degrees.
Understanding Common Prescription Conversions
| Original Sphere (S) | Original Cylinder (C) | Original Axis (A) | New Sphere (S’) | New Cylinder (C’) | New Axis (A’) |
|---|---|---|---|---|---|
| +1.00 | +0.50 | 90 | +1.50 | -0.50 | 180 |
| +2.00 | +1.00 | 180 | +3.00 | -1.00 | 90 |
| -1.00 | +0.25 | 45 | -0.75 | -0.25 | 135 |
| -2.00 | +0.75 | 120 | -1.25 | -0.75 | 210* |
| +0.50 | +0.50 | 60 | +1.00 | -0.50 | 150 |
* For the New Axis (A’) values that exceed 180, subtract 180 to keep the axis within the standard range of 1 to 180 degrees. For example, an original axis of 120° with an addition of 90° becomes 210°, which is beyond the standard range, so we subtract 180° to get 30°.
Example of Plus to Minus Cylinder Calculator
Consider a prescription with an original sphere (S) of +2.50 diopters, a cylinder (C) of +1.00 diopter, and an axis (A) of 90 degrees. Using our calculator:
- The new sphere (S’) would be +3.50 diopters.
- The new cylinder (C’) would be -1.00 diopter.
- The new axis (A’) would adjust to 180 degrees (90° + 90°).
This example illustrates the straightforward nature of converting prescriptions using the Plus to Minus Cylinder Calculator.
Most Common FAQs
Conversion ensures compatibility across different lens crafting systems and preferences among eye care professionals, facilitating accurate lens production and optimal vision correction.
Yes, the principles apply equally to eyeglasses and contact lenses, although it’s crucial to consult with an eye care professional for specific considerations related to contact lenses.
The calculator is highly accurate, provided the input values are correct. It’s a reliable tool for professionals and a helpful guide for patients seeking to understand their prescriptions better.
