A Guide for Reading Eyeglasses Prescription

Getting a spectacles prescription is the first step towards improving your vision. After an eye examination, your optician might mention that you are long-sighted, short-sighted, or you have astigmatism. So what do the numbers on the prescription mean? What do the abbreviated terms mean in full? In this post, you will learn how to decipher the various parts of a spectacles prescription so that you can discuss the prescription knowledgeably with an optical shop or optician near you when buying glasses.  

What are OD and OS?

To understand a prescription, you must first know what the abbreviations mean. The two main abbreviations you will find are OD and OS. OD stands for oculus dexter, whereas OS stands for oculus sinister. These are both Latin terms for right and left eye. On the prescription, you might see a column marked OU. In full, OU stands for oculus uterque, which is Latin for ‘both eyes’.

The abbreviated Latin terms are a tradition for prescription written for eye medicines, contact lenses and spectacles. There are, however, opticians that modernize the abbreviations to RE (right eye) and LE (left eye).

Other details on an eye prescription

  • SPH (Sphere)

This is a term used to indicate the amount of lens power. It is measured in diopters (D) and used to correct long-sightedness or short-sightedness. If the number under the SPH has a plus sign, it implies you are long-sighted and vice versa for a minus sign. The term Sphere means the correction of your eyesight is ‘spherical’ or the same in all meridians of your eye.

  • CYL (Cylinder)

This is an indication of lens power used to correct astigmatism. If there is nothing under this column, it shows you don’t have astigmatism, or your astigmatism is slight and needs no correction with lenses. ‘Cylinder’ means the lens power that is added to correct astigmatism will not be spherical but shaped in a way that one meridian doesn’t have an added curvature, whereas the meridian that is perpendicular to it has maximum power as well as lens curvature for correcting astigmatism. The digit in the CYL column may have a plus or minus sign depending on whether the lenses are correcting long-sighted astigmatism or short-sighted astigmatism. The cylinder power follows sphere power in a spectacles prescription. 

  • Axis

This is for the lens meridian with no cylinder power for correcting astigmatism. Axis is defined with a number ranging from 1 to 180, where 90 corresponds to the eye’s vertical meridian, whereas 180 corresponds with the horizontal meridian. In case your prescription has cylinder power measurement, it should also have an axis value that comes after CYL power and is preceded by ‘X’ when written in freehand. The axis is the lens meridian located 90 degrees from the meridian containing cylindrical power.

  • Add

This refers to added magnifying power applied at the bottom area of a multifocal lens when correcting presbyopia. Even if there is no plus sign preceding the number, the number is always a ‘plus’ power. This ranges from +0.75 to +3.00 D. The number is similar for both eyes.

  • Prism

This refers to prismatic power. It is measured in p.d (prism diopters) or superscript triangle. This is meant to make up for issues with eye alignment. A very tiny percentage of spectacle prescriptions will include prism. Prism direction is indicated using four abbreviations: BD= base down, BU=base up, BO=base out and BI= base in.

Cylinder power, add power and sphere power all appear in diopters. They are presented in the form of decimal and written ins quarter-dioptres increment. That is increments of 0.25 D. For the axis, these are whole numbers ranging from 1 to 180. These numbers signify meridional location and not power. If prism diopters are presented in decimal form, only a single digit will appear after the decimal, such as 0.5.

At times the optician can write a specific lens recommendation on the prescription. This includes photochromic lenses, anti-reflective coating, and progressive lenses. This is done to help you get a comfortable vision correction.

Glasses prescriptions are not the same as contact lens prescriptions

This is an important point to remember when reading a prescription. Spectacle prescriptions are for buying spectacles only. If you need contact lenses, you need a contact lens prescription.

After an eye examination, your optician will provide you with a written prescription. Once you get this, feel free to get in touch with us. At Optician on Wheels, we will come to your home and office. We make it easy for you to get your lens and frames. Call us now to find out more.