How To Read Your Eyeglass Prescription

What is a diopter? What does "add" mean? What do "OS" and "OD" stand for? How do I read this prescription? Here is a guide to those abbreviations and numbers that will help.

What Are Diopters?

Vision is measured in diopters, which are the standard unit of measurement in eyeglasses. These are usually expressed as "-1.00" or "+1.50" on the prescription.

A negative number here indicates myopia (nearsightedness) and a positive number indicates hyperopia (farsightedness). But what precisely IS a diopter? A diopter is the distance at which you are able to see objects clearly and dictates the strength of the lens.

For instance, a -1.00 diopter means you can see objects clearly at 1 meter, however anything past that is fuzzy. A -2.00 diopter means you see clearly at ½ meter with further objects getting fuzzy.

Inversely, a positive diopter, like +1.00 indicates that you see clearly at 1 meter, but closer items get fuzzy.


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Abbreviations

Here is a list of abbreviations used commonly on eyeglass prescriptions:

• OS - Oculus Sinister or "left eye"

• OD - Oculus Dexter or "right eye"

• OU - Oculus Uterque or "both eyes"

• SPH - Sphere or the power of the lens in diopters.

• CYL - Cylinder indicates the power of the lens for astigmatism only. Nothing written means no astigmatism.

  Negative numbers indicate nearsighted astigmatism; positive numbers mean farsighted astigmatism.

• AXIS - Describes a sector of the eye requiring no cylinder power for astigmatized vision correction. Imagine a protractor centered on the pupil. This is how the sectors, or meridians, are determined.

• ADD - This is used to indicate the use of bifocal (multifocal) lenses. This is done for presbyopia patients. The lens has two powers. One for normal vision and one for up close detail work, like reading.

• PRISM - Measured in diopters, prismatic power helps with eye alignment trouble.

  Prism direction orientation is in relation to its thickest edge, or base. This will be abbreviates as:
  

  • BU - Base up
    
  • BD - Base down
    
  • BI - Base inward
    
  • BO - Base outward

  As Prism is only prescribed for people with muscle or focusing disorder, you probably won't even see it on your prescription.

Sample Prescription:

OD -2.00 SPH +2.00 add 0.5 p.d. BD
OS -1.00 -0.50 x 180 +2.00 add 0.5 p.d. BU

A bottom magnifier (+2 diopters) is added to correct presbyopia and a 0.5 prism direction with orientation BASE DOWN.

The left eye has a small amount of astigmatism and needs -1 SPHERE POWER to correct myopia and -0.5 AXIS POWER at 180 degrees to correct astigmatism, a presbyopia correction (+2 diopters) and a 0.5 prism direction with orientation BASE UP.

Pupil Distance

The other factor involved in this is pupil distance. This is the distance between the pupils as you are looking far away.

This measurement is extremely important top of your eyeglass prescription so that the exact center of the lenses are directly lined up with the pupils.

If this is measured wrong, the result is an incorrect prism effect that can cause eyestrain. Usually, PD can be requested from the doctor or you could call the last optician that made glasses for you. They should know your PD.

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