Positive Cylinder Format
The second way is not very common and it’s called positive (or plus) cylinder format. In plus cylinder’ format, the cylinder number is always positive. Very few optometrists use this format but many ophthalmologists are trained to write prescriptions this way. So depending on who does your eye exam, you might end up with a prescription in plus cylinder or minus cylinder format.
Example:
Sphere | Cylinder | Axis | |
Rx: | -4.75 | +1.25 | 90 |
If you notice that there is a plus sign in front of the cylinder numbers in your prescription and you’re interested in ordering glasses or contact lenses online, you’re going to have to get your eye doctor to convert it from plus cylinder to minus cylinder.
Converting Plus Cylinder To Minus Cylinder
This can be done in 3 steps. For educational purposes, I will describe the process below, but remember that only your eye doctor is qualified to do this for you.
Let’s take this prescription for example:
Sphere | Cylinder | Axis | |
Rx: | -3.00 | +2.00 | 090 |
Converting The Sphere
In this step your eye doctor will calculate the new Sphere number by taking the sum of the original Sphere and Cylinder numbers. Knowing how to add positive and negative numbers is critical to getting this step right.
Using our example prescription…
Since -3.00 + (+2.00) = -1.00,
Sphere | Cylinder | Axis | |
Rx: | -3.00 | +2.00 | 090 |
becomes…
Sphere | Cylinder | Axis | |
Rx: | -1.00 | +2.00 | 090 |
Converting The Cylinder
To convert the Cylinder, your doctor will flip the Cylinder’s sign. The Cylinder number itself stays the same, but the sign changes from a ‘plus sign’ to a ‘minus sign’.
Using our example prescription from the step above…
Sphere | Cylinder | Axis | |
Rx: | -1.00 | +2.00 | 090 |
becomes…
Sphere | Cylinder | Axis | |
Rx: | -1.00 | -2.00 | 090 |
Converting The Axis
To convert the axis, your doctor must perform some addition/subtraction. If the original axis is anything between 001 and 090, 90 will be added to the original axis. If the original axis is anything between 91 to 180, 90 will be subtracted from the original axis.
Continuing with our example…
Sphere | Cylinder | Axis | |
Rx: | -1.00 | +2.00 | 090 |
becomes…
Sphere | Cylinder | Axis | |
Rx: | -1.00 | -2.00 | 180 |
If the axis is under 90 degrees, then add 90 degrees (ex: 70+90= 160 degrees)
The end result of a conversion from positive cylinder to negative cylinder will usually end up looking very different from the original. Nevertheless, it does represent the exact same prescription
Some Other Examples
Sphere: -4.50, Cylinder: +1.50, Axis: 045 becomes… Sphere: -3.00, Cylinder: -1.50, Axis: 135
Sphere: +3.25, Cylinder: +0.75, Axis: 120 becomes… Sphere: +4.00, Cylinder: -0.75, Axis: 030
Sphere: plano, Cylinder: +1.25, Axis: 179 becomes… Sphere: +1.25, Cylinder: -1.25, Axis: 089
Reference & Courtesy of:
https://contactsadvice.com/2015/07/convert-plus-cylinder-to-minus-cylinder/
