Here’s everything you ever wanted to know to understand your glasses and contact lens prescriptions.
If you wear both glasses and contact lenses, you might have wondered if your prescriptions are interchangable. Could you submit an eyeglass prescription to get a prescription for contact lenses filled, for example, and vice versa?
The answer is no. While they both have the same objective — to correct what eye doctors call “refractive errors” that prevent you from seeing correctly — prescriptions for eyeglasses and contact lenses are distinctly different.
FIRST, WHAT ARE REFRACTIVE ERRORS?
Refractive errors are a group of vision problems that occur when the shape of your eye doesn’t bend light correctly. Instead of seeing clearly, what you see is a blurred image.
Refractive errors are very common and include nearsightedness (myopia), farsightedness (hyperopia), presbyopia (aging of the eye lens), and astigmatism (when the curvature of the cornea or natural lens are asymmetrical).
HOW ARE THE TWO TYPES OF PRESCRIPTIONS DIFFERENT?
Part of the reason for the differences is that contacts are placed directly on the lens of your eye, while glasses sit about 12 millimeters away. That seemingly minor difference requires a different prescription strength for each type of corrective eyewear.
Your contact lens fits the size and shape of your eye, so it includes more information about lens curvature and width.
Here is one example of a glasses prescription (note that the prescription format used by your eye doctor may appear different):
HERE IS ONE EXAMPLE OF A CONTACT LENS PRESCRIPTION (NOT THAT THE PRESCRIPTION FORMAT USED BY YOUR EYE DOCTOR MAY APPEAR DIFFERENT):
|SPH / Power||CYL||Axis||BC||DIA||Lens Name|
|OD||-1.25||-0.75||170||8.6||14.0||Acuvue Oasys for Astigmatism|
|OS||-0.75||-1.25||20||8.6||14.0||Acuvue Oasys for Astigmatism|
WHAT DO THE ABBREVIATIONS AND SYMBOLS ON BOTH PRESCRIPTIONS MEAN?
Some of the abbreviations and symbols you’ll see on your eyeglasses prescription:
- OD (oculus dexter) refers to the right eye. OS (oculus sinister) refers to the left eye. You may also see the abbreviation OU, which refers to both eyes. All are Latin abbreviations.
- SPH stands for sphere, or the amount of lens measured in diopters (the unit used to measure your vision correction).
- CYL stands for cylinder, indicating the amount of power you need to correct astigmatism. (Astigmatism is present when your cornea has a more oblong shape, rather than a round shape.)
- Axis is what tells you the orientation of the astigmatism.
- ADD stands for addition, and is used in bifocals to add correction for farsightedness.
- PD indicates the pupillary distance, or the distance between the centers of your pupils.
- A minus (-) sign indicates you are nearsighted.
- A plus (+) sign indicates you are farsighted.
- The further from zero the number is on your prescription, the stronger your prescription is.
In addition to many of the above, your contacts prescription will include the power, base curve and diameter, as well as the contact lens material and brand or type of contacts.
- Power is the same as sphere (SPH).
- BC is the abbreviation for base curve, the measurement of the curvature of the back surface of the contact lens. It’s determined by the shape of your cornea.
- DIA stands for diameter, and indicates the size of your lends. It works with the base curve to determine how your lens will fit.
- The contact lens material and/or brand is indicated because different lens materials have different degrees of oxygen permeability. Your eye care professional will know which one is best for you.