Deciphering your Eyeglass Prescription
You have just walked out of the eye doctor office after an eye exam with a piece of paper. When you first look at it, noticed that has a variety of numbers and letters scribbled on it but have no idea about what they mean.
Well, first of all, this piece of paper is your eyeglass prescription.
Before thinking about what kind of cool eyeglasses to choose, your mind is still trying to decipher the meaning of this array of characters.
Whether you're a long time glasses wearer or a newbie, knowing how to read your eyeglass prescription will give you a better understanding of what it could say about your vision and overall health. Also, what will happen in case your eye doctor mentioned that you are nearsighted or farsighted, or if you have astigmatism.
This article will help you decipher all parts of your eyeglass prescription and pinpoint the important numbers you need to know when discussing your eyewear choice with an optician.
The components of an Eyeglass Prescription
It will have the doctor's name and address printed on it plus your name and the prescription's expiration date (typically one or two years from the date of your exam). Your doctor's signature will also appear at the bottom.
Now, for all those characters in between:
OD vs OS
The first step to understanding your eyeglass prescription is knowing what "OD" and OS" mean. They are abbreviations for oculus dexter and oculus sinister, which are Latin terms for "right eye" and "left eye".
Your eyeglass prescription also may have a column labeled "OU." This is the abbreviation for the Latin term oculus uterque, which means "both eyes".
Though the use of these abbreviated Latin terms is traditional for prescriptions written for eyeglasses, contact lenses and eye medicines, some doctors and clinics have opted to modernize their prescriptions and use RE (right eye) and LE (left eye) instead of OD and OS.
On your eyeglasses prescription, the information for your right eye (OD) comes before the information for your left eye (OS). Eye doctors write prescriptions this way because when they face you, they see your right eye on their left (first) and your left eye on their right (second).
The lens power prescribed to help you see distant objects clearly.
This indicates the amount of lens power, measured in diopters (D), prescribed to correct nearsightedness or farsightedness. If the number appearing under this heading has a minus sign (-), you are nearsighted; if the number has a plus sign (+) or is not preceded by a plus sign or a minus sign, you are farsighted.
The term "sphere" means that the correction for nearsightedness or farsightedness is "spherical", or equal in all meridians of the eye.
This indicates the amount of lens power for astigmatism. If nothing appears in this column, either you have no astigmatism, or your astigmatism is so slight that it is not really necessary to correct it with your eyeglass lenses.
The term "cylinder" means that this lens power added to correct astigmatism is not spherical, but instead is shaped so one meridian has no added curvature, and the meridian perpendicular to this "no added power" meridian contains the maximum power and lens curvature to correct astigmatism.
The number in the cylinder column may be preceded with a minus sign (for the correction of nearsighted astigmatism) or a plus sign (for farsighted astigmatism). Cylinder power always follows sphere power in an eyeglass prescription.
This describes the lens meridian that contains no cylinder power to correct astigmatism. The axis is defined with a number from 1 to 180. The number 90 corresponds to the vertical meridian of the eye, and the number 180 corresponds to the horizontal meridian.
If an eyeglass prescription includes cylinder power, it also must include an axis value, which follows the cyl power and is preceded by an "x" when written freehand.
The axis is the lens meridian that is 90 degrees away from the meridian that contains the cylinder power.
This is the added magnifying power applied to the bottom part of multifocal lenses to correct presbyopia. The number appearing in this section of the prescription is always a "plus" power, even if it is not preceded by a plus sign. Generally, it will range from +0.75 to +3.00 D and will be the same power for both eyes.
Pupillary Distance (PD)
The distance between the centers of each pupil (black hole). Once you reach adulthood, this number does not change. The PD number is very important for properly fitting lenses and is required for optimum visual comfort and performance. The measurement of the proper PD enables the center of the lens to align with the center of the pupil.
This is the amount of prismatic power, measured in prism diopters ("p.d." or a superscript triangle when written freehand), and prescribed to compensate for eye alignment problems. Only a small percentage of eyeglass prescriptions include a prism.
When present, the amount of prism is indicated in metric units (0.5, for example), and the direction of the prism is indicated by noting the relative position of its "base" or thickest edge. Four abbreviations are used for prism direction: BU = base up; BD = base down; BI = base in (toward the wearer's nose); BO = base out (toward the wearer's ear).
Sphere power, cylinder power and add power always appear in diopters. They are in decimal form and generally are written in quarter-diopter (0.25 D) increments. Axis values are whole numbers from 1 to 180 and signify only a meridional location, not a power. When prism diopters are indicated in decimal form, typically only one digit appears after the period (e.g., 0.5).
Your eye doctor also might write specific lens recommendations on your eyeglass prescription -- such as anti-reflective coating, photochromic lenses and/or progressive lenses -- to give you the most comfortable vision correction possible.
How is a Contact Lens Prescription Different?
Your eyeglass prescription can only be used for the purchase of eyeglasses. A contact lens prescription, which is also written by your doctor, will contain specific information such as base curve, lens diameter, and the specific brand and maker of the contact lens.
Also, the power of an eyeglass prescription frequently is modified when determining the best contact lens power. One reason is that eyeglass lenses are placed some distance (usually about 12 millimeters) from the surface of the eye, whereas contact lenses rest directly on the eye's cornea.
An accurate contact lens prescription can be written only after a contact lens fitting has been performed and the prescribing doctor has evaluated your eyes' response to the lenses and to contact lens wear in general.