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Eye exams are pretty straight-forward: a patient goes to the doctor, the doctor measures eyesight, and the patient is out the door. One thing that a lot of people do not understand, however, is the prescription that one gets from the eye doctor. What do all of those numbers mean?

Most people do know that 20/20 vision is perfect. It means that they are neither near-sighted nor far-sighted. Someone with 20/20 vision is in the normal range, because they can see and read images and text correctly from 20 feet away from the target. The higher the numbers go, the worse the eyesight will be. For example, if a person’s vision is 20/40, that person is only able to see at 40 feet what a person with perfect vision can see at 20.

You might also see other indications on your prescription, including the following:

You may find the acronyms OD and OS. These are the letters that indicate ‘right’ in Latin (OD) and ‘left’ (OS). Some doctors simply use RE and LE to indicate the right or left.
The sphere (SPH) measurement will tell your eyeglass maker what strength of lens to use to make your glasses or to dispense the correct contact lenses. If the number has a minus sign, you are diagnosed as being near-sighted. A plus sign indicates far-sightedness.

Cylinder (CYL) indicates a measurement of astigmatism. You will only see this indicated on your prescription if you actually have an astigmatism that is severe enough to need correction. If an astigmatism is detected, the CYL indication will be followed by an axis measurement number that will also help with your glasses or contacts.

The two final indications that you may see on your prescription are ‘add’ and ‘prism.’ ‘Add’ defines the strength of magnification with regards to bifocals or other multi-focal lenses, and ‘prism’ will tell the person making your glasses of any adjustments that the lenses will need to correct for any eye alignment problems you may have.

If you have any questions about your eyeglass prescription, your doctor or the person who is making your glasses are sure to have an answer.