In South Africa, we get an eye examination done by an optometrist or ophthalmologist and usually we purchase a pair of glasses from the same store. Many of us never bother to ask for the examination results/prescription. If you have, good for you, because a month from now your puppy might have a knibble on your glasses. Having your prescription on hand, saves you a call to the optometrist and also opens the door to shopping around for the best deals.
But what do all those numbers on the prescription actually mean?
Here's what a typical prescription looks like:
|OS:||+0.75 -1.50 x 85|
OD (oculus dextrus) and OS (oculus sinister) refers to right (OD) and left (OS) eyes. Often an optometrist will simply use the abbreviations R and L. Prescriptions will always have a + or - sign in front of the digits. This indicates whether you are farsighted (+) or nearsighted (-). Patient X in this example is therefore farsighted.
The right eye requires a correction of +1.25 DS. DS (dioptre sphere), where dioptre refers to the unit of measure for refractive power. The sphere part refers to whether the eye is spherical and no astigmatism is present. I'll get to that now.
In the left eye, astigmatism is present. This means that the lens/cornea of patient X is not perfectly spherical. In these cases the eye needs correction in two planes, hence the two numbers. +0.75 D correction and a -1.50 D astigmatism correction at a plane of 85 degrees. For the sake of not getting too technical, people who suffer from astigmatism usually experience blurry vision up close and at distance. As appose to just receiving blurry vision near or far - in once instance.
ADD refers to addition power. As we get older, our eyes lose the ability to change focus effectively. This means patient X needs bifocals or multifocals to allow clear vision at distance and near. The lens therefore needs to be 1.25 DS stronger in both eyes, when patient X needs to see up close. This typically happens after 45 years of age and is called hyperopia.
PD refers to pupillary distance. It is the distance between your pupils when looking straight ahead. In patient X's case it is 65mm. This distance is important, because when fitting prescription lenses to a frame, the centre of lens needs to be aligned with the wearer's pupil. If not, headaches and distortions could be the result, instead of clear vision. (Keep this in mind the next time you buy over-the-counter readers at the pharmacy.)