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Reading Your Eyeglass Prescription

So you’ve had an eye exam and you’ve received your first eyeglass prescription. It may be imposing to see it written on the same sheet of paper as your medicine.  You don’t want to simply hand your prescription over to your optician. Understanding your eyeglass prescription is as important as knowing about your prescription medication.

Before jumping into the more technical terms we should examine some of the more important parts of your prescription. In order for it to be valid, your prescription must have an expiration date and your doctor’s signature, usually with his or her title (D.O., M.D., etc.).

Rx prescription for corrective glasses

A prescription usually expires after a year or two. The younger you are the more likely your eyes are to change rapidly. Because of this, a teenager or young adult’s prescription is usually a year or less. Typically, your set expiration date will be when your doctor suggests you should get your eyes re-examined.

Now onto the core of the prescription. OD and OS are the unknown terms that jump out first. They are abbreviations for the Latin terms oculus dexter and oculus sinister. These terms describe your right eye and left eye respectively.

OU can also appear on your prescription. This is the Latin term oculus uterque, or both eyes.  This term is most frequently used to describe your vision with both eyes uncovered, or combined vision. However, it can also be used to describe both eyes at once if they have the same measurements.

Some doctors have chosen to drop the tradition and use RE and LE for right and left eye respectively. BE (both eyes) is also used.

OD normally comes before OS on your prescription. When your doctor is facing you to exam your eyes your right eye appears on his or her left, and thus is customarily examined first.  

The top row of your prescription, usually marked “Distance,” contains the measurements used to help you bring distant objects into focus. This is sometimes abbreviated as “DV,” or distance vision.

The first measurement is “Sphere.” This indicates the refractive power of the lens used to correct near- or farsightedness. A negative sign indicates nearsightedness, while a positive sign indicates farsightedness. For all of these measurements, a lack of a positive or negative sign normally indicates a positive measurement.

Sphere is measured in a unit called diopters (D). Common measurements range from -3.00 to +3.00 D. Doctors normally prescribe in increments of 0.25 D. Sphere is used when the correction for near- or farsightedness is equal in all parts of the eye. If no correction is needed the measurement is 0.00. This is sometimes abbreviated as “pl.” for the Latin word for plane, plano. 

When the correction is not uniform (due to astigmatism for example), “Cylinder” is used in addition. This column could be blank, indicating that you don’t have astigmatism or that it is slight enough to remain uncorrected. Cylinder is measured in the same way that Sphere is. 

If there is a Cylinder correction, there must also be a measurement for Axis. Axis measures the location of the eye where the Cylinder correction needs to be applied. It is measured in meridians which is analogous to degrees, and ranges from 0 to 180. 180 is the on the side closest to your nose, and 0 is opposite that. You may see this measurement proceeded with an “x.”

Below the Distance section, there may be an Add section. This section is sometimes abbreviated as “NV,” or near vision. This is for correcting presbyopia in the bottom part of multifocal lenses. This is again measured in diopters and normally ranges between +0.75 and +3.00 D; it is always positive and is usually the same for both eyes.

“Prism” is a rare addition to a prescription. It is used to correct eye alignment problems. It is measured in prism diopters, abbreviated as p.d. or Δ, in increments of 0.5 p.d. There are four abbreviations that could appear, indicating where the base of the correction is. This indicates the direction the correction is pointing. BU (Base Up) points towards you forehead, and BD (Base Down) points downward. BI (Base In) points towards your nose, and BO (Base Out) points toward your ear.

In bifocal lenses, the top row denotes the measurements for the top half of the lens and the bottom row denotes the measurements for the bottom half.

picture of PD measurement

With high enough corrective lens, “PD” is also used. This is pupillary distance, the distance in millimeters between the resting positions of the pupil in each eye to the nose.

Also at high corrective lens is “BVD.” This is back vertex distance, or the distance in mm from the back of the lens to the cornea. These measurements are needed at higher prescriptions because a more precise lens is needed in order to reduce blur. 

Sometimes your doctor may put additional notes on your prescription. He or she can add a recommendation for a specific brand or material. He or she may also add notes for the type of lens, like adding a tint or anti-reflective coating.

It is important to note that an eyeglass prescription and a contact lens prescription are not interchangeable. Different measurements are used for each because they are placed at different lengths from the eye. You may, however, use your eyeglass prescription to get prescription sunglasses in addition to your eyeglass.

Remember that your doctor is legally required to give you a prescription after an eye exam. You are allowed to take your prescription to an optician of your choosing.