Understanding Your Glasses Prescription,A Plain Language Guide

Most patients leave an eye appointment with a piece of paper containing numbers and abbreviations they cannot interpret. This is not ideal,understanding your prescription helps you make informed decisions about glasses, contact lenses, and your eye health. Here is a plain language explanation of every value you are likely to see on your glasses prescription.

The Basic Structure

A glasses prescription typically has two sections: OD (Oculus Dexter,the right eye) and OS (Oculus Sinister,the left eye). Some prescriptions use R and L instead. Each eye has its own set of values. A third value, OU (Oculus Uterque), sometimes appears for measurements that apply to both eyes together.

SPH (Sphere)

The SPH value corrects the basic focusing error of the eye. A negative (−) number means the eye is short-sighted (myopic),you see better up close than at distance. A positive (+) number means the eye is long-sighted (hyperopic) or presbyopic,additional plus power is needed to focus. The number is measured in dioptres (D). A value of 0.00 or "Plano" means no spherical correction is needed.

CYL (Cylinder)

The CYL value corrects astigmatism,an irregular curvature of the cornea or lens that causes vision to be blurred or distorted at all distances. It is always written with its companion value, the AXIS. If there is no CYL value, the prescription has no astigmatism component. CYL can be written as positive or negative depending on the convention used by the optometrist,both mean the same astigmatism, expressed differently.

AXIS

The AXIS tells the lens laboratory the angle at which the astigmatism correction must be orientated in the lens. It is expressed as a number between 1 and 180 degrees. It only has meaning in combination with the CYL,a CYL value without an AXIS is incomplete.

ADD (Addition)

The ADD value appears on prescriptions for reading glasses, bifocals, and progressive (multifocal) lenses. It is the additional plus power added over the distance prescription to help the eye focus at close range. ADD values typically range from +0.75 to +3.50D, increasing with age. A value appears only if near correction is needed (usually from the mid-40s onward, when presbyopia develops).

PD (Pupillary Distance)

The PD is the distance in millimetres between the centres of the two pupils. This measurement is used by the optical lab to position the optical centres of the lenses correctly in front of your pupils. An incorrect PD can cause eye strain, double vision, and discomfort. PD is sometimes given as a single number (binocular PD,both eyes together) or as two numbers (monocular PD,each eye measured separately from the centre of the nose).

Prism

A prism correction (if present) addresses misalignment between the two eyes,a binocular vision issue. It is expressed in prism dioptres (pd or △) with a direction (Base In, Base Out, Base Up, Base Down). Most prescriptions do not include prism.