Understanding your eyeglass prescription

An eyeglass prescription is an order written by an eyewear prescriber, such as an optometrist or ophthalmologist, that specifies the value of all parameters the prescriber has deemed necessary to construct and/or dispense corrective lenses appropriate for a patient.


The parameters specified on spectacle prescriptions vary, but typically include the power to which each lens should be made in order to correct blurred vision due to refractive errors, including myopia, hyperopia, astigmatism, and presbyopia. It is typically determined using a phoropter asking the patient which lens is best, computer automated refractor, and through the technique of retinoscopy. Opticians are not eye doctors and, therefore, are not licensed to write an eyeglass prescription. A dispensing optician will take a prescription written by an optometrist or ophthalmologist and order and/or assemble the frames and lenses to then be dispensed and sold to the patient.


Understanding the prescription

• OD – Right Eye: usually the top value in an eyeglasses prescription chart. OD is a latin abbreviation for oculus dexter, which means "right eye." The measurements in the OD row are thus for the right eye.
• OS – Left eye: usually the bottom value in an eyeglasses prescription chart. The measurements in the OS row are for the left eye. OS in Latin is oculus sinister, or "left eye."
• SPH – Sphere: the horizontal curve of the eye’s lens; (+ or – number between –10 and +10) Also called your prescription’s power, this is how strong your lenses need to be.
• CYL – Cylinder: indicates astigmatism; (+ or – number between –4 and +4 ) If you have a number for CYL, you, like many glasses wearers, have astigmatism. This simply means the vertical curve of your eye is shaped more like a football than a sphere.

Remember: AXIS goes with CYL. If you have a CYL measurement, you will always also have an AXIS measurement. Likewise, if you don’t have a CYL measurement, you will not have an AXIS measurement.

• AXIS:  Indicates the degree and direction of astigmatism; (+ number only between 1 and 180) Often this number will be proceeded by an x. You will not have an AXIS if you don’t have a CYL.
  (Note: If a word is in your CYL or AXIS section with no number, you do not have astigmatism; this is there to trick you. We’re watching your back.)
• PRISM: The majority of glasses wearers will not have a PRISM measurement. If you do, you have a more difficult glasses prescription that requires an optician’s expertise. If there is prism the prism base also should be mentioned.
• ADD: Only for bifocals/progressive lenses (+ number only, usually up to 5.00) How much magnifying power you need in your bifocal/progressive lenses. 
  (Note: In some prescriptions, this is written in a NV or near-vision section, while the rest of the prescription is written in the DV or distance-vision section.)

Tips:

1. Pay particular attention to whether the SPH and CYL are positive or negative.
2. A normal single-vision eyeglasses prescription without astigmatism only needs the SPH. You will also need your Pupillary Distance (PD), which may be included on your prescription as two numbers; add these numbers together and you’ve got your PD. If your prescription doesn't include your PD, you have a few options. First, you can call your optometrist and they may have it and provide it to you. You can also go to a local optician and have them measure it. Or, you can measure it yourself.
3. If a number is not in the CYL and AXIS spaces, there is no value for either.
4. You will only have an ADD measure if you wear bifocals/progressives.
5. SPH, CYL, and ADD are written with decimals. Sometimes there are no decimal points on the prescription. You then get to add them in. 
   (Example: a SPH written as “175” is really “1.75”; a CYL written as “12” would be “1.20”; and an ADD written as “25” would be “2.50.”)
6. Sometimes, the prescription will not be written on an actual grid. The numbers will, however, likely be written according to the grid shown, with OD on top, OS on bottom, with columns SPH, CYL, AXIS, etc.
7. If you see the word PLANO in the row of boxes for one eye, you need a “balance lens” for that eye. A balance lens is a non-prescription lens; you need one if only one of your eyes requires vision correction.
8. Last, but not least, if both of your eyes have the same prescription, only one row of numbers may be written on your prescription.

credits: buyglasses-online, wikipedia, justeyewear