Eyeglasses Prescription Terms: What Do They Mean?
Here’s how to decode all those letters and numbers on your prescription and know what the values mean.
7
min read
Reviewed on
August 27, 2024
Written by
Katherine Solem
Expert review by
Jonathan Gula
Here’s how to decode all those letters and numbers on your prescription and know what the values mean.
August 27, 2024
Written by
Katherine Solem
Expert review by
Jonathan Gula
Here’s how to decode all those letters and numbers on your prescription and know what the values mean.
August 27, 2024
Written by
Katherine Solem
Expert review by
Jonathan Gula
Have you ever taken a close look at your eye prescription? Covered in sets of letters, numbers, and plus and minus signs, it can seem like a secret code. But understanding it can help you make the right choices about your vision care.
Here’s how to decode your prescription and know what those values mean.
OD is short for oculus dexter, which means right eye in Latin. Some prescriptions use “R” for the right eye instead.
OS is short for oculus sinister, which means left eye in Latin. Some prescriptions use “L” for the left eye instead.
This indicates how much correction you need. A minus sign (-) in front of the number means nearsightedness (difficulty seeing faraway objects). A plus sign (+) means farsightedness (difficulty seeing close objects).
How strong your glasses are, called lens power, is measured in units called diopters. So you may see the letter “D” after those numbers (like -3.00 D). The bigger the number in either direction, the more vision correction you need.
This measures astigmatism, which means your eye is shaped a little differently. Typically, the eyeball is round (spherical). But in some people, the cornea or lens is shaped in a way that causes the eyeball to be more oval (cylindrical). Both eyeglass lenses and contacts can help correct vision in people with astigmatism.
Like the sphere value, the cylinder number can be either negative or positive. The greater the number in either direction, the more vision correction you need.
If your prescription doesn’t have any numbers in the cylinder column or says “DS,” you don’t have astigmatism in either eye.
This means no astigmatism. Some people write DS in the Cylinder column instead of leaving it blank.
The Axis indicates where the astigmatism sits on your eye. It is measured in degrees from 1 to 180. If you have a number in the cylinder column of your prescription, you will also have an axis number. Likewise, you will not have an axis number if you have no cylinder number.
Eyes are supposed to move at the same time and in the same direction. When this doesn't happen, the image each eye sees doesn't line up with each other, causing double vision. A prism corrects double vision. It is added to a certain place in the lens and orients the image as it comes into your eye so that it lines up with the image from the other eye.
The prism will have its own corrective power measured in diopters. Sometimes, that number may have a triangle symbol next to it. The triangle simply represents the shape of a triangle prism and how it bends light.
The base indicates that you need prism correction. The prism is placed in a certain position and orientation in the lens. If you need prism correction, your prescription will include one of the following to indicate how the prism should be positioned. The base is the thickest edge of the prism.
This adds magnification to bifocal, trifocal, or progressive lenses. If you need help seeing things up close but don’t want separate reading glasses, magnification can be added to the lower part of your lens.
This is the distance between the center of your pupils (the black part of your eye). Pupillary distance tells where to center the vision correction in each lens. PD can be indicated as one value: the distance between your two pupils. Or it can be written as two values, each expressing the distance between the center of your pupil to the bridge of your nose. Faces aren’t perfectly symmetrical or mirror images, so it’s normal to have slightly different pupillary distances between your left and right eyes.
You’ll need your PD value to order glasses with us. So be sure to ask your eye doctor for that number. If they don’t provide it, don’t sweat it. Learn how to measure your pupillary distance at home.
All prescriptions have an expiration date. For eyeglasses, the expiration date is typically two years from the date of your eye exam. It could be shorter if your prescription is changing rapidly or you have a health condition that can affect your vision. In that case, your eye doctor may set a shorter expiration date so that you return for evaluation sooner. In some states, eye prescriptions are only good for one year.
