Deciphering Your Eyeglass Prescription

Following an official eye exam with an optometrist, you are usually handed a copy of your eyeglass prescription, which contains information that a lab requires to make a pair of glasses to correct your unique vision problems. This prescription often appears rather cryptic, however, so read on if you would like to know once and for all what those numbers and letters really mean! The following basic table format of an eyeglass prescription will serve as an example:


O.D. and O.S.

The first thing to note is that there are two main rows that span the columns, one marked “O.D.” and the other marked “O.S.”. These are abbreviations for the Latin terms ocular dexter (which means “right eye”) and ocular sinister (“left eye”). The optometrist examines and records the values for each eye separately, because your two eyes can potentially be very different in terms of their refractive error and therefore correction.

SPH or “Sphere”

This value indicates whether you are nearsighted (myopic) or farsighted (hyperopic) and to what extent. If you see a minus sign in front of the number, it means you have myopia and if you see a plus sign, it means you have hyperopia. The actual value of the number written here represents the amount of lens power required to correct the blurriness in your vision caused by your refractive error. The higher this number is, the more powerful a lens you will need to have in your eyeglasses in order to see well.

CYL or “Cylinder” and AXIS

If you have a number in the CYL box, it indicates that you have some astigmatism in that particular eye. Astigmatism is a visual distortion caused by an irregularly shaped cornea, and the value here indicates the power of lens required to correct any blurriness specifically due to astigmatism. If you have a value in a CYL box, there will always be a value in the AXIS box as well because the two go hand-in-hand. The axis value describes the location and direction of your astigmatism, and helps the lab position your lenses at the right angle within the frame.


If you are over the age of 40, you may see a number in this box which represents the “reading addition” that should be added to your prescription to account for presbyopia. Presbyopia often leads to a difficulty reading small print as you age, and the ADD number tells the lab how much extra strength you need in your glasses to be able to see up close at ‘reading range’, in addition to any other corrections for near or farsightedness.


It is fairly rare to see a number in this box, but if you do it may indicate that you have some kind of eye muscle imbalance that prevents your eyes from functioning well as a pair. A value in the PRISM box indicates the need for additional correction via a prism in the lens, which will allow you to see more uniformly and comfortably. A value for BASE will only appear when there is a value for PRISM, because it tells the lab making your glasses where exactly to place the base edge of the prism within your lens (either up, down, in or out).

Now that you can decipher your eyeglass prescription, be sure to visit Woodhams Eye Clinic’s newly expanded optical shop to find that next great pair of glasses, custom made to correct your particular refractive errors.

For questions or comments, please contact Woodhams Eye Clinic.

Image Source: Liz Officer

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