Definition of Eyeglass Prescription
How to Read an Eyeglass Prescription Intro:
There are literally an infinite number of iterations and combinations of eyeglass prescriptions that are out there and prescribed by optometrics, opthamologists and retina experts. This article will help explain and dissect what exactly each of the numbers on your prescription mean, and why you don't really need a trained professional to interpret it for you.
The First Number: Sphere
Here is a basic lens prescription for someone who is nearsighted.
SPR CYL AXIS
OD -1.25 0.00 000
OS -1.50 0.00 000
First thing first, OD means "right eye". OD is an acronym for a latin phrase that no one cares to remember. And likewise, OS means "left eye"
The sphere in this case is -1.25 for the right eye, and -1.50 for the left eye. There is an important point to be made. Regardless of if there is a + or - before the sphere number, the rule of thumb is, the higher the number, the worse the eyesight. So a -6.50 is always worse than a -2.50, and a +7.50 is always worse than a +2.50. Ignore the plusses and minuses for the moment.
So, in this situation above, the left eye is worse than the right. It's a marginal difference, but if we were to assume -1.25 for each eye, the left eye would be straining slightly to see as clearly as the right, and that could in turn, cause further issues, such as headache, fatigue, and migraines.
Now, what about the plusses and minuses from before?
For the sphere, which is essentially (ignoring astigmatism, macular degeneration, or need for a bifocal) your ability to see objects far away, and in some cases, up close. Minus prescriptions mean you are nearsighted and are able to see objects close up (and for some of the highest prescriptions, such as -15.00, only what is right in front of their face), and Plus prescriptions mean that you have trouble seeing what's in the distance (beyond ten feet), and anything close-up, but middle ground is marginally better.
So, say a person with a minus prescription wanted to read a book, they would tend to have it right in front of their eyes, like the proverbial bookwork. A person with a plus prescription would have the book at arm's length and try to read it like that (not always possible due to other complications).
Now you know what the sphere property of the prescription is. It is the curvature of the lens, and corresponding thickness of the lens to amplify the vision to achieve the desired effect. -10.00 prescriptions tends to be very thick, especially around the edges of the lenses, while a +10.00 prescription will be very thick and bubbled in the middle of the lens. It's all about correcting the issue.
The Second and Third Numbers, Cylinder and Axis
The cylinder is a measure of one's astigmatism. Astigmatism is an abnormality in the shape of the eye that causes light entering the retina to become distorted and sent in the wrong direction. People with heavy astigmatism mimic those with heavy plus sphere prescriptions. They cannot see far away or close up. It's a level of haziness and blurriness that will cause a person to perpetually try to be focusing on something that they will not be able to see clearly. A person with heavy astigmatism will have eyes that look like footballs rather than soccer balls. Luckily, this is something that can be corrected with eyeglasses.
In the previous example, there was no number listed for astigmatism. This means that the patient does not require any correction in that area. But, let's use a new example.
SPR CYL AXIS
+5.25 -3.25 65 (OD)
+4.25 -4.00 110 (OS)
In this example, both eyes are farsighted, and have heavy astigmatism. It is extremely rare to find a patient with -4.00 astigmatism, and this patient in particular would be essentially blind as a bat without glasses. The cylinder twists the sphere about an axis, causing the lens shape to change in nature, and curvature. What that cylinder and axis are, are determined during your optical exam.
The lenses in your glasses compensate for your eyes' natural defect by changing the direction in which the light comes through the lens, and at what angle and magnitude. So, people with high spheres, cylinders and axis rely on glasses and contacts way more than say, someone with just a basic -1.25 prescription. They are, at that point, essential to functioning in everyday life.
Cylinder always is displayed in a negative number (the higher the negative number, the worse the astigmatism), unless you are seen by an opthamologist. They have the option of writing it as a positive number, but there's a mathematical formula we Opticians follow to convert their positive annotations to the correct negative ones.
The axis is always on the range from 0 to 180, where 0 and 180 are basically equivalent. It indicates how much the lens much be adjusted about the axis to correct which way the light enters the eye. Low numbers and high numbers are essentially equivalent. There is no "bad" axis number, only different ones.
Finally, Bifocal Additions
In the following prescription, the bifocal prescription is written as a +x.xx after each of the other numbers, and most doctors either specify either PAL or FT for their prescriptions, which mean no-line or line bifocals, respectively.
Take the following prescription as an example:
Eye SPR CYL AXIS ADD
OD -1.50 -1.00 88 +2.50
OS -1.75 -1.25 82 +2.50
In this scenario, they have mildly bad eyesight distance-wise, with their -1.50 and -1.75 sphere, and some hindrance from their -1.00 and -1.25 astigmatisms, and then there's the ADD portion.
In this case, the bifocal wearer needs a strong bifocal of +2.50 which is essentially just like a magnifying glass in the way that it amplifies things up-close, but is useless at a distance. The doctor specified PAL which means they need a progressive bifocal, or that's their preference, or both.
When it comes to progressive bifocals, there is a small area at the very bottom of your lens where you can utilize that +2.50 magnification. Don't confuse +2.50 and +2.50 of a sphere. They're completely unrelated. A +2.50 sphere with a +2.50 reading addition, does not equal +5.00 magnification.
The bifocals are meant for objects within 5 feet, but usually more around the 3-feet and closer range. It magnifies whatever you're looking at, and makes up for your eye's natural tendency to become unable to focus on objects up-close.
Be thankful if you only have a +1.50 or +1.75 reading addition, as that means you could technically get by without glasses for reading, but it would be difficult. You'd end up having to put the book at arm's length and squinting.
Many people who do not have high spheres and no astigmatism but have problems with reading up-close, go for reading glasses, which serve the same exact purpose, minus the correction of the other eye problems.
Other Miscellaneous Facts
Prisms are a rare occurrence in the optical world because simply put, not many people need this specific correction. What prisms are, are specially built in lens adjustments that compensate for a specific eye issue. This eye issue is if one of your eyes, or both, are not in-line with each other, as in they cannot focus on the same object. For lazy-eye folks, prisms are useless, but for those people who by birth defect have an eye looking slightly off in a different direction, prisms correct the double vision that naturally occurs.
Sometimes Doctors are Over-Zealous on their Prescription Pads
They will often start to prescribe what types of lenses you need (see my other lens article), and what perks you need on your glasses to function. Most of the time this is just a scam to get sales for the optical shop, but sometimes there are honest optometrists who have your best interest in mind. These doctors usually leave that sort of business for Opticians and other trained staff that can assist you in the process of buying glasses.
Eyeglass Prescriptions are Universal
Sometimes they will omit this fact at doctors offices so that you stay there and buy what they have to offer, but in reality, you can take your prescriptions (and you ARE ENTITLED to your prescription) anywhere and everywhere, so don't forget that.
Closing Thoughts on Reading an Eyeglass Prescription
Hopefully this article has helped explain a bit about what an eyeglass prescription is, and how to read/decipher it. I'm sure, by now, you're calculating just how bad your eyes really are. I'm just happy and glad that modern science has gotten optometry and eyecare down to such a solid routine. Sure, yearly exams are usually required, and sure, eyeglasses fall apart pretty easily, but there are very few secrets in the field of optometry, and as a consumer, and patient, you have a right to know what they are, and make informed decisions based upon that data. I hope this article has been helpful and wish you all the best of luck in your eyecare endeavors!