Positive cylinder conversion

Eyewear prescription positive cylinder conversion

Astigmatism prescriptions can come either with positive (+) cylinders or negative (-) ones depending on the instruments used to measure and cut the lenses and also sometimes this depends on the country from where you have your prescriptions. Most of the prescriptions given in the UK will be with a negative (-) cylinder, which you will be able to input when placing an order of spectacles.

We currently use in our system positive (+) cylinders, which means that any prescription with a negative (-) cylinder needs to be converted to the positive (+) equivalent. You don't need to worry about the quality of the vision correction that you were prescribed, as this is not influenced after the conversion. The conversion will change the details of the lenses you will see in the invoice you will receive together with your spectacles, but it will result in the vision correction you were prescribed.

Below, you can see the rules of conversion, to know what to expect to see in the invoice in case you place an order on our website with a negative (-) cylinder.

Add the sphere and cylinder values together algebraically taking the positive and negative signs into account. The result is your new sphere power. Now change the plus sign of the cylinder to minus, and keep the cylinder power the same. Finally, change the axis by 90 degrees.

-2.50 -0.75 080 becomes and is equal to -3.25 +0.75 170

+4.75 -1.25 120 becomes and is equal to +3.50 +1.25 030