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What is a prescription?

The prescription you are given after your eye examination is the formula that will be used to make your spectacles or contact lenses and it is usually written in the following format:

Sphere / Cylinder / Axis / Prism Power / Prism Base / Reading Addition

For example, compare the following two prescriptions:

+3.00DS/-0.75DC x 180 2? In +1.50D

-2.50DS/+1.25DC x 75 +1.00D

The first eye is longsighted (hyperopic) as shown by the + sign in front of the sphere number and the second eye is shortsighted (myopic) as shown by the – sign in front of the sphere.

Both eyes have astigmatism because they have a cylinder number. For simplicity imagine the front lens of the eye is shaped more like a rugby ball than a football.

In the first prescription the astigmatism is horizontal i.e. the flatter curve of the rugby ball is horizontal, whereas the flatter curve is vertical in the second eye. This is shown by the axis number – imagine holding a protractor in front of the eye and rotating the rugby ball around the different angles where 180 is the zero (or horizontal) line and 90 is the vertical line.

The first prescription shows a prism correction – in this example the ‘base in’ prism will help the eye to converge and read comfortably with its fellow. Only a small percentage of patients, less than 1%, will have prism incorporated into their prescription.

Finally, both prescriptions have a reading addition which is the focussing power that is added to the sphere to create the reading lens. Eyes under 40 years of age rarely need a reading add.

It should be noted that eye prescriptions can be written in two forms – positive and negative cylinder. For example, the following two prescriptions are exactly the same, but in different form

-2.00 / -1.00 x 180

-3.00 / +1.00 x 90


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