Understand your eyeglass prescription
Understanding Your Eyeglass Prescription You recently had your eyes examined and you are ready to receive a new prescription. They gave him a sheet of paper, with some numbers, when he was coming out of his exam. Wondering what those numbers mean?
Your eyeglass prescription consists of the powers of the lenses necessary to give you the best possible vision.
What the numbers mean
Your prescription will consist of numbers that represent the powers of the lenses necessary to correct the vision in each of your eyes. A typical eyeglass prescription will look like this:
OD -3.25 -1.25 x 175
SO -4.25 -1.00 x 165
The first line of an eyeglass prescription is, by convention, for the right eye. It is sometimes indicated as OD. The left eye or the SO is the second line of your glasses prescription. The first column number is the spherical power of each eye. It is indicated by numbers known as diopters, usually in steps of 0.25. Every now and then you may see .12 steps, but this is rare. The powers of the spheres are identified by a plus or minus sign. This is very important to know because a less power (-) corrects the vision problem, myopia and a more power (+) corrects the hyperopia. So if the power of the sphere of your prescription is a negative number, you are nearsighted and if the power of the sphere is a positive number, then you are farsighted. The higher the number, the greater the degree of myopia or hyperopia.
The second column of your glasses prescription is called the cylinder power. If there is a number in this column, it means 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 an sph. or sphere written in this column. In any case, there is no energy, so you do not have astigmatism. It is possible to have astigmatism in one eye and not the other.
The last column of your prescription is the axis. This represents the direction or location of your cylinder stem. If there is no cylinder power in your glasses prescription. which cannot and will not have an axis number indicated. If you have a cylinder power, then you will have an axis. This is a measure in degrees from 0 to 180. Only half of the 360 degree circle is used because 90 and 270 would be identical.
So if you know the power of your sphere’s power cylinder and the axis of each eye, you will know if you are nearsighted, farsighted, or have astigmatism.
Occasionally, a prescription may have a prism power. This is usually for people suffering from diplopia (double vision). It can be a fraction or a decimal such as 1/2 or 0.75. If there is a power prism, it must also have a direction or a base. This is up, down, in or out. The prism is relatively rare.
A multifocal prescription like bifocal, trifocal or progressive lens will also have additional power. This power is in addition to the sphere, cylinder and shaft and is indicated on the next line of the prescription. It is always a positive number like +1.25 or +2.25. This is the amount of additional focusing power to allow for near reading or vision.
The last number needed to fill a prescription for glasses and make a new pair of glasses is the pupillary distance, also known as “PD.” It is the distance, in millimeters, between the centers of the pupils of the eyes, for example 62. This is described in more detail as PD distance and near PD, such as 63/60. Distance PD is when you are looking straight ahead and Near PD is when you are looking closely. When you look closely, your eyes turn inward, therefore the near PD will always be less than the distance PD. Each eye can also be measured individually. This is called monocular PD. It would be expressed as 31/30 or 31/31 depending on its symmetry and facial characteristic. An adult PE varies little over time.
NOTE: A written eyeglass prescription may or may not contain a PD measure.
Make sure your prescription is up to date and that your eyes are checked regularly to make sure you are seeing as clearly as possible. The eyes change gradually over time and it can be so gradual that you may not be aware of it. Routine eye exams also detect silent thieves of sight, such as glaucoma and cataracts.
Did you know that you have a legal right to obtain a prescription for your glasses from your eye care professional? That’s how it is. Request your prescription at the time of your eye exam or request it later, it’s your right.