Frame Size GuideYour optical prescription is an important document that helps lab technicians understand how to make and size your eyeglasses and sunglasses so that your vision is properly corrected. However, since it's used by technicians, it's full of technical jargon and abbreviations.

If you're struggling to understand your prescription, the following is an explanation of the most common terms found on an optical prescription.

First, the most common question people ask is:



This can be confusing. But if you remember where your vision is clearest, it's easy to get it straight.

If you can see things close-up very clearly, but have trouble reading road signs or a blackboard when you're sitting at the back of the class, you are near-sighted. Your nearer vision is better.

If you have trouble reading or threading a needle, but can see the road signs or the blackboard, you are far-sighted. Your far-off vision is better.


"OD" and "OS" are the most basic terms on the prescription. OD stands for oculus dexter—that's Latin for "right eye". OS stands for oculus sinister, or the left eye. Some doctors may opt to use abbreviations in English (so "RE" and "LE").

Sometimes, you may see "OU" on your prescription. This means oculus uterque, or "both eyes". It's used to indicate an identical condition or adjustment to be made in the lenses for both eyes.


Sphere indicates the main lens power needed to correct your vision. Sphere is measured in dioptres (D). If you're near-sighted, the sphere measurement will begin with a minus-sign. If you're far-sighted, the number will have a plus sign (or no symbol at all).

Positive lenses for far-sighted patients concentrate light, while negative lenses for near-sighted patients spread the light out.


Cylinder indicates the finer-tuned lens correction needed for individuals with astigmatism. Astigmatism means that the front part of your eye is not perfectly curved—one part is flatter than the other. This can cause blurred vision. Astigmatism can occur in the cornea or the lens of your eye.

Like sphere measurement, cylinder is measured in diopters and has a minus for near-sighted prescriptions and a plus sign for far-sighted prescriptions (or no symbol at all). If you don't see any number in this area, you don't have astigmatism.

Cylinder power always follows sphere power in an eyeglass prescription.


Lenses bend or refract light to correct your vision's focus. The higher the number in the sphere or cylinder measurement, the more drastically the lenses affect your vision.


Lenses that correct astigmatism will need to define an axis—the area 90 degrees away from the part of the lens that contains the cylinder adjustment to correct the astigmatism. This can be a number from 1 to 180. The axis identifies a location, not a strength, so there will not be a plus or minus sign.


"Add" stands for added magnification. This is for wearers of bifocals or progressive lenses, and tells the lab how much magnification to add to the bottom part of the lens. Generally, additional magnification ranges from +0.75 to +3.00 D, and will be the same in both eyes.


If your eyes are mis-aligned, the optometrist will indicate a prismatic power to help account for this problem so that the corrective area of the lens matches where your pupil looks. Very few people have a prescription with a prism adjustment.

Prism direction is indicated with four abbreviations: BU = base up (towards the wearer's forehead); BD = base down (towards the chin); BI = base in (toward the wearer's nose); BO = base out (toward the wearer's ear).