Understand and Calculate your Eye Prescription

Your eye prescription will explain whether you are:

• Short-sighted

• Long-sighted

• Have astigmatism

To understand your eye prescription, you need to know what these abbreviations and numbers mean.

OD, OS and OU or ODS

A prescription is first abbreviated by the Latin terms for the eyes. These abbreviations are as follows:

• The term OD refers to the right eye, or oculus dexter.

• The OS stands for oculus sinister, or the left eye

• OR or ODS stands for oculus uterque, or both eyes.

Sphere (SPH)

An individual’s SPH is the amount of lens power required to clearly see. It is an indication of how long-sighted or short-sighted they are. A number follows the measurement of correction (diopters). You should generally use a stronger correction the closer the number is to 0.

When you have a minus sign (–) on your SPH number, you are short-sighted. Myopia (short-sightedness) indicates that you are able to clearly see objects close to you. Objects further away will appear blurry or difficult to see. The greater your short-sightedness, the more minus the number.

Having a plus sign (+) on your SPH number indicates that you are longsighted. You will be able to clearly see objects that are farther away than those that are close up with long-sightedness (hyperopia).

Cylinder (C or CYL) and Axis

An eyeball with astigmatism is not completely round, causing light to bend evenly and form a clear image on the back of the eye when it enters your eye. Because the cornea is irregular for those with astigmatism, it does not have a spherical curvature, but rather a toric one (like a doughnut in a ring). As light rays radiate from objects in the field of vision, they don’t always “land” at the same point, resulting in distorted vision and a greater degree of out of focus in some directions than in others.

Astigmatism is described in three parts in a prescription. The strength of your astigmatism is described in the first section. The irregular curve is described in the second section.

• Sphere (SPH): as explained above, this indicates whether you are long-sighted or short-sighted.

• Cylinder (CYL): This is the amount of lens power that is required to correct astigmatism. It can be positive or negative. The higher the number, the stronger your astigmatism. A CYL number will not be available if you do not have astigmatism, or if it is too small to need correction.

• Axis: A positive number between 0 and 180 degrees that acts as a map of your eye, explaining the location of the irregular curve affecting your vision.

Astigmatism is treated by concomitantly adjusting the CYL and Axis measurements. If you do not have astigmatism, only the SPH measurement will appear on your prescription. If you do have astigmatism, SPH, CYL, and AXIS measurements will appear on your prescription.

ADD, Prism and Pupillary Distance (PD)

The addition of lens power is necessary to make reading easier. It appears on prescriptions for reading glasses or the bottom part of varifocal lenses to correct natural age-related longsightedness (presbyopia). For both eyes, it is usually the same and the correction ranges between +0.75 and +3.00 diopters.

As an eye alignment problem arises, prism power determines how much prismatic power is required. The prism diopters (PDDs, triangles) are also used to indicate where to place the prism on the glasses using BU = base up; BD = base down; BI = base in (towards the nose); BO = base out (towards the ear). This method does not apply to all prescriptions.

PD is a critical component of your prescription as it indicates where your lenses should have their optical center so that you receive the best vision possible. In order to fit your prescription to the spectacle frame, PD is important. It measures the distance between the pupils in your eyes (the black circles).

Contact lens prescription

How to read a contact lens prescription

If you wear contact lenses, your prescription may differ from that of your glasses, owing to the fact that contact lenses are fitted directly on the surface of your eyes, while glasses sit on your nose, a few millimetres away.

The following components make up a contact lens prescription:

• A spherical number will always be present, and sometimes an astigmatic correction may also be present,

• The base curve of a contact lens is defined as the curvature of the lens. It usually ranges from 8 to 10 degrees.

• The diameter of the lens is its size.

The specification will also include information about the manufacturer, brand, and expiration date of your lenses.

Please consult your optometrist if you are still unsure how to read glasses or contact lens prescriptions.

Other notes

In addition to prescription lenses, your eyeglasses or contact lenses may also have other features, such as photochromic lenses (which become lighter or darker in response to the lighting), scratch-resistant coatings, or anti-reflective coatings for use on the computer.