What are All These Numbers??

A single Vision prescription for distance, with no astigmatism prescription.  In the industry, we refer to this as a 'spherical prescription'.  The SPH notation indicates there is no astigmatism correction needed.

A single vision prescription for distance with astigmatism correction.  This one is written in - cyl.

Often a patient will come to me with prescription in hand, and ask, "How much did my prescription change?" or, "What do all these numbers mean?"  It's not easy to make sense of the small intricacies of your eyeglass prescription without a little background information.  I will try to get only as technical as I need, but please feel free to ask questions in the comment section.  Lets start out with a simple prescription:

You can see above there are two rows and several columns on your prescription.  The top row is labled OD (the abreviation for the latin word for 'right eye'), all of the numbers across the top row pertain to your right eye.  The OS column, then, pertains to your left eye.  The right eye will almost always be listed first.  So lets examine the columns then.  The column labeled "SPHERE" is where you will see whether a prescription is needed for distance ( indicated with a minus sign AKA 'nearsighted') near, (plus sign) or neither, indicated with the letters PL, standing for plano or zero.  This is representing the power of correction needed in order to clear things up.  The person with the perscription on the left needs glasses to see in the distance.  But there are numbers in the other columns as well.
The CYL and AXIS mean that this person also has astigmatism that needs to be corrected.  A lot of people become concerned when I mention they have astigmatism and exclaim, "No one ever told me I have that!"  Truth is, a large portion of the prescriptions I see in my office have astigmatism.  It simply means your eyeball isn't perfectly spherical (round) and that causes the light to split up when it enters your eye, instead of staying in one nice, straight line. 
The CYL column indicates the amount of astigmatism correction needed to clear things up, and can be written in either + form (usually by ophthalmologists) or in - form, like the photo above, which is the way most optometrists write their prescriptions.  When the prescription is in + Cyl form, it must first be converted to - cyl form in order for you to discern whether your prescription is truly + or -.  Here is an example of two different ways to write the same prescription:
+.75 -1.75x180

-1.00 +1.75 x90

You might have noticed that the middle number, the CYL stayed the same, but the sign changed.  To switch between the + cyl form and the - cyl form of the prescription, you simply agebraically add the CYL power to the sphere power, switch the sign of the CYL and change the axis by 90 degrees.  Lets say, for our example above, you were handed a prescription written in the + cyl form and needed to transpose it to the - cyl form.  We would first algebraically add the +1.75 + (-1.00) = +.75 (in optical, if there is no sign in front of the number, it is assumed that it is +) The amount of CYL stays the same, 1.75, but we change the sign from + to -.  The last step then, is to change the axis.  If the existing axis is 90 or more, bract 90 from it.  If the existing axis is 89 or less, we would add 90 to it.  Since our rx is 90, we would add 90 to it.  So our end result is the top prescription. +.75 -1.75x180.  Where the first number represents the SPHERE column, the second the CYL and the third(preceded by and x) indicates the AXIS. 

Still with me?  If you get your prescription in + cyl and don't feel like transposing it, any optician should be able to do this for you easily.  I am always willing to do this for you at no charge, just leave a comment. 

Now then, the AXIS.  This pertains to how we are directing the light, and it runs from 1 to 180 degrees.  Your axis generally doesn't fluxuate very much after puberty, so if you see a dramatic change( more than 15-20 degrees) from one prescription to the next, expect to feel a little disoriented when you pick your new pair.  Note: it is possible to have a prescription only for astigmatism. the prescription would look something like this:

Note the "PL" in the SPHERE column?  That means ther is no correction for farsightedness or nearsightedness, only astigmatism.

I'm going to skip ahead to the ADD box, on some prescriptions this may also be called NV for near vision.  If you have a number filled in here, it means you need bifocals.  I felt a few of you shuddering at the thought!  It is actually just a natural part of life.  There is a group of muscles in the eye (the ciliary body) that is responsible for adjusting the lens of the eye to focus on things near and far.  About the age of 40, most people notice that muscle starting to lose its ability to pull things into focus, particularily close up things or tiny writing.  This is the point where we start to hold things a little farther from our face, in order to get a proper focus.  Eventually, the muscle weakens to the point where we need a corrective lens to do what the muscles no longer can.  Unfortunately, these muscles can't be trained or exercised, but there are new options for corrective surgery for those who can't stand the thought of wearing bifocals.  The bifocal, reading or Add power will always be a + number, because it is magnifying what you are looking at.  I will discuss types of bifocals in another post. 

The other column on your prescription (sometimes abbreviated as Rx) is PRISM.  If you have numbers and symbols here, it means that there is some sort of muscle imbalance that is causing double vision.  Your doctor may prescribe a press-on prism (or, Fresnel prism) as a way to test out how it works and how you adapt before we grind the prism into your lens.  A prism redirects the way the light enters the eye, and helps pull your eye muscles in the desired direction to help you focus as a singular object instead of double objects. 





A bifocal prescripton with Prism and astigmatism, written in plus cyl.

There can be many different combinations.  Some people only need glasses for reading so the entire SPHERE, CYL and AXIS portion simply says PL SPH with an ADD.  If you are lucky enough to have a prescription like this, you can buy over-the-counter reading glasses (AKA 'readers') very inexpensively.  For all other prescriptions, however, it is best to see your optician to have your prescription filled.