Glasses prescriptions to the untrained eye can appear to be a random sequence of numbers and letters, but once you understand what each number means you can understand how to read this code.

Put simply, the numbers represent diopters, a unit used to describe the amount of correction an eye needs, which is often seen as an abbreviated D in a prescription but is often omitted. Generally speaking, the further away from zero these numbers are, the more correction the lens requires. These numbers are listed under the letters OS and OD. OS stands for oculus sinister, which means it is for the left eye, while OD stands for oculus dextrus, which is the right eye. If you see the letters OU, this involves both eyes.

The first numbers in the series will also have positive or negative signs attached to them. A plus sign signals the eye is far-sighted (known as hyperopia), while a minus sign signals the eye is near-sighted (known as myopia). This first number in the sequence is known as the spherical number. If you have astigmatism, there will be a second and third number. The second number is the cylinder number, which measures the astigmatism and can be positive or negative.  In most cases it written in the negative form. The third number, known as the axis, describes the curve in the eye with a number between 1 and 180 and describes the orientation of the eye.In the case of no refractive error, you will see a 'pl', which stands for plano or plano shp. A fourth number, the ADD, may be included, especially for patients over the age of 45.

This means the prescription will be different for distance and closer focus, such as reading. The number measures the additional correction needed to focus on close objects.Finally, if a PD (pupillary distance) is included in your prescription, this just means the distance between your eyes. This can be an important figure to help your dispensing optometrist or optical advisor best fit the lenses correctly.