Understanding Your Eyeglass Prescription You’ve recently had your eyes examined and you are ready for some new prescription eyeglasses. You were handed a piece of paper, with some numbers on it, as you left your examination. You are wondering what do those numbers mean?
Your eyeglass prescription consists of lens powers needed to give you the best vision possible.
What the Numbers Mean
Your prescription will consist of numbers that represent the lens powers needed to correct the vision in each of your eyes. A typical eyeglass prescription will look something like this:
OD -3.25 -1.25 x 175
OS -4.25 -1.00 x 165
Add +2.25
The first line of an eyeglass prescription is, by convention, for the right eye. It is sometimes noted as O.D. The left eye or O.S. is the second line of your eyeglass prescription. The first column number is the spherical power for each eye. It is indicated by numbers known as diopters usually in .25 steps. Occasionally you may see .12 steps but this is rare. Sphere powers are identified by either a plus or minus sign. This is a very important to know because a minus (-) power corrects the vision problem nearsightedness and a plus (+) power corrects farsightedness. So if the sphere power of your prescription is a negative number you are nearsighted and if the sphere power is a positive number than you are farsighted. The larger the number the greater the degree of nearsightedness or farsightedness.
The second column of your eyeglass prescription is called the cylinder power. If there is a number in this column that means that you have astigmatism. The same plus and minus number conventions as the sphere power apply to the cylinder of your prescription. There may be no number in this column or there may be a sph. or sphere written in this column. In either case there is no power so you have no astigmatism. It is possible to have astigmatism in one eye and not your other.
The last column of your prescription is the axis. This represents the direction or location of your cylinder power. If there is no cylinder power in your eyeglass prescription. than it cannot and will not have an axis number indicated. If you have a cylinder power then it will have an axis. This is a measurement in degrees from 0 to 180. Only half of the 360 degree circle are used because 90 and 270 would be identical.
So if you know your sphere power cylinder power, and axis for each eye, you know if you are nearsighted, farsighted, or have if you have an astigmatism.
Occasionally a prescription may have a prism power. This is usually for people who suffer from diplopia (double vision). It may be a fraction or a decimal such as 1/2 or .75. If there is a prism power it must also have a direction or a base. This is either up, down, in, or out. Prism is relatively rare in occurrence.
A multi focal prescription such as bifocal, trifocal, or progressive lens will also have an add power. This power is in addition to the sphere, cylinder, and axis and is indicated on the next line of the prescription. It is always a positive number such as +1.25 or +2.25. This is the amount of extra focusing power to enable reading or near vision.
The last number need to complete an eyeglass prescription and fabricate a new pair of eyeglasses is the pupillary distance otherwise know as known as “PD”. It is the distance, in millimeters, between the centers of the pupils in your eyes, for example 62. This is further described as distance PD and near PD such as 63/60. Distance PD is when you are looking straight ahead and near PD is when you are looking close. When you look close your eyes turn in, therefore the near PD will always be less than your distance PD. Each eye can also be measured individually. This is called a monocular PD. It would be expressed as 31/30 or 31/31 depending on your symmetry and facial feature. An adult PD varies little with time.
NOTE: A written eyeglass prescription may or may not contain a PD measurement.
Make sure your prescription is current by having your eyes examined regularly to insure you are seeing as clearly as possible. Eyes do change gradually over time and it may be so gradual that you do not notice. Routine eye examinations also screen for the silent stealers of sight such as glaucoma and cataracts.
Did you know that you have a legal right to obtain your eyeglass prescription from your eyecare professional? That’s right. Request your prescription at the time of your eye examination or request it later, it’s your right.
