Understanding your eyeglass prescription
If you wear glasses or contacts you may have looked at your prescription and wondered what do all those numbers mean?
This article will help you decipher all parts of your prescription and discuss it knowledgeably with an optician when you're buying eyeglasses.
What Does OD and OS mean?
The first step into understanding your is knowing what OD and OS means. OD is Latin for oculus dexter meaning right eye, and OS is Latin for oculus sinister meaning left eye. Did you know that Romans believed that anyone who wrote with their left hand was being controlled by the devil, hence why left eye is oculus sinister. Some opticians have modernized and converted the OD to R.E.(right eye) and OS to L.E.(left eye).
Other terms on your Prescription:
Sphere(sph) : 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.
Cylinder (CYL): 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.
Astigmatism is caused by a deviation from spherical curvature, which results in distorted images. If you have an astigmatic cornea you may vision blurry vision and letters on a page may be double or have shadows.
Axis: 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 your prescription includes a Cylinder reading it will always have a Axis.
Add: 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.
If you wear Bifocals or Progressives you may also need a measurement called Seg. Height. This measurement measures the distance from the bottom of a frame to your pupil. This area is normally your "reading prescription" and the rest is your distance prescription. This measurement will be different for different types of frames, lens size, and your particular need.
Prism: This is the amount of prismatic power, measured in prism diopters, prescribed to compensate for eye alignment problems. Only a small percentage of eyeglass prescriptions include prism. 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).
You may need prism is you are suffering from double vision. Prism moves the eyes focal point up, down or side to side to correct the vision.
Sample Eye Glass Prescription:
SPH CYL AXIS ADD
OD -2.00 +2.00 add
OS -1.00 -0.50 180 +2.00 add
This prescription corrects the myopia(nearsightedness) in the right eye. The left eye is corrected for myopia and astigmatism. Both eye will have an addition reading power added in for their bifocals or progressives.
An Eyeglass Prescription is NOT a Contacts Lens Prescription
Eyeglass and contact lens prescriptions aren't the same. An eyeglass prescription is for the purchase of eyeglasses only. It does not contain certain information that is crucial to a contact lens prescription and that can be obtained only during a contact lens consultation and fitting.
A contact lens prescription will contain addition information such as base curve, wearing schedule, and lens material. These will all be determined during a contact lens fitting. Additionally if you are a first time wearer of contacts you will also be trained on how to wear contacts and care for them.
Your eyeglass prescription is yours to Keep and to Keep up to date
No eye doctor can keep your eyeglass prescription hostage. You can take this prescription anywhere you like.
An eyeglass prescription also will either have a 1 or 2 year expiration date that is designated by the Doctors office. Make sure you keep your prescription up to date as optical will not fill an expired prescription.