Converting Plus Cylinder to Minus Cylinder for Prescriptions

Converting a prescription with a Plus Cylinder component to a Minus Cylinder form


A couple of weeks ago, two friends asked me how to convert a prescription with plus cylinder to minus cylinder form. With the opening up of travel, eye care practitioners may get more overseas visitors coming in with their own prescriptions.


Hence, this article seeks to help you convert prescriptions from plus cylinder to minus cylinder form. Perhaps it may also help those who have come to Singapore from other countries that use the plus cylinder form of writing to understand why their Singapore prescription may look different.



Plus and Minus cylinders in a prescription


We can have a plus or minus cylinder written in a prescription.


For instance:


An example of a minus cylinder prescription: +2.00/-1.00x180

An example of a plus cylinder prescription: +2.00/+1.00x180


In Singapore, we typically use the minus cylinder form of writing.



Converting the prescription


So how do we convert from a plus cylinder to a minus cylinder?


Step 1) Add the Sph and Cyl together to get the new Sph power

Step 2) Change the Cyl to minus (negative)

Step 3) Turn the axis by 90 (note: axis can be 001 to 180)


Let’s try some examples:


-1.00/+1.25x150

Step 1) -1.00 and +1.25 will give a new Sph of +0.25

Step 2) +1.25 will become -1.25

Step 3) Axis of 150 will become 060

Therefore, the minus cylinder prescription will be:

+0.25/-1.25x060


+2.00/+0.50x180

Step 1) +2.00 and +1.25 will give a new Sph of +2.50

Step 2) +0.50 will become -0.50

Step 3) Axis of 180 will become 090

Therefore, the minus cylinder prescription will be:

+2.50/-0.50x090


-5.00/+2.00x020

Step 1) -5.00 and +2.00 will give a new Sph of -3.00

Step 2) -2.00 will become -2.00

Step 3) Axis of 020 will become 110

Therefore, the minus cylinder prescription will be:

-3.00/-2.00x110



Simple!