How to Read Your Spectacle Prescription
You’ve just finished having your eyes checked and was given a prescription for eyeglasses. You know that the numbers given are used to correct your vision from blur or eyestrain, but what does it all mean?
Looking at your basic prescription elements
Each optometrist will have their own format for writing a prescription so yours may not look exactly like the one pictured above, but all the same information should be there:
1) Patient Name and contact information.
2) The first abbreviations you will notice are the letters OD, OS, and OU.
OD = right eye
OS = left eye
OU = both eyes.
3) Pupillary distance or PD is the distance from the centre of one eye to the centre of the other eye. The values in the "Far" column are for distance only prescriptions and the values in the "Near" column are used for reading only prescriptions. Some opticals or online stores will ask for one measurement for both eyes (the OU value) or the measurement for each individual eye (OD for the right eye measurement and OS for the left eye measurement).
4) The overall prescription. The first number in a prescription is the spherical correction, often abbreviated as SPH. This is a measure (in dioptres) of how powerful your eyeglass lenses needs to be to correct either nearsightedness (noted with a - ) or farsightedness (noted with a + ).
The second number is the cylindrical correction. This will only be on a prescription if you have astigmatism. If you do have astigmatism, cylinder (CYL) will indictate the amount in dioptres and AXIS will indicate where it's located relative to your eye.
ADD is the additional magnifying power applied to the bottom part of bifocal or multifocal lenses to correct presbyopia (the normal loss of near focusing ability that occurs over the age of 40). The number appearing in this section of the prescription is always a "plus" power and is the same power in both eyes.
5) Exam Date and Prescription Expiration Date. It's important to take note of the prescription expiration date, filling out an expired prescription may lead to eyestrain especially if your eyes have changed from when you had your last eye exam.
There may be other notes on your prescription related to lens design or coatings that your optometrist has specifically recommended to give you the most comfortable vision correction possible.
There is a strong misconception that a glasses prescription is the same as a contact lens prescription. Contact lens prescriptions will indicate the brand of contacts that are appropriate for your eyes. Size measurements such as base curve (BC) and diameter (DIA) are also indicated on contact lens prescriptions. Please don't use your glasses prescription to order contact lenses, by doing so, you may be ordering lenses that are too strong which will lead to eyestrain and headaches while doing close work. I always indicate on spectacle prescriptions that if you need a contact lens prescription, get a proper contact lens exam.
That’s it! You can now read your own prescription, and understand what it reveals about your vision.