Convert Plus Cylinder To Minus Cylinder

Prescriptions for glasses and contact lenses are written using certain conventions. Depending on the prescribing doctor's training and preferences, a prescription may be written in negative cylinder format (the most common format), or in positive cylinder format.

Any prescription can be written in either of these two formats. If you are planning to order glasses or contact lenses online (with a valid prescription), you may be confused when it comes to entering the numbers in your prescription if you were given a prescription in positive cylinder format.

For more information about how to read a glasses or contact lens prescription, click here.

All online vendors of glasses and contact lenses use the negative cylinder format. In this article you will learn how to identify a prescription in positive cylinder, and how your doctor changes your prescription from one format to another.

Negative Cylinder Format

If your prescription has any number under the 'Cylinder' section, you have some amount of astigmatism. Astigmatism can be described numerically in two different ways.

The first way is the most common way and it's called negative (or minus) cylinder format. In negative cylinder format, the cylinder number is always negative. This is the format that most optometrists and opticians use when prescribing glasses and contact lenses.

Example:

	Sphere	Cylinder	Axis
Rx	−3.50	−1.25	180

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

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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

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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

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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

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

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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

Now you are familiar with the difference between a prescription in positive cylinder and in negative cylinder. If you notice that your prescription is in positive cylinder, make sure you bring it to your eye doctor to have it converted before you attempt to use it to order glasses or contact lenses online.