2024-03-15
That Piece of Paper They Handed You
You just had an eye exam and the doctor handed you a prescription that looks like it was written in another language. Numbers with plus and minus signs, abbreviations that mean nothing to you, maybe some handwriting that is barely legible. You nod, shove it in your pocket, and hand it over at the optical shop without knowing if those numbers are good, bad, or somewhere in between.
Let us fix that.
The Basics: OD and OS
OD stands for "oculus dexter" which is Latin for right eye. OS stands for "oculus sinister" which is Latin for left eye. Some newer prescriptions use RE (right eye) and LE (left eye) instead, which makes a lot more sense, but most doctors stick with the Latin.
Your prescription will have a row for each eye. They are almost always different from each other. That is normal. Your two eyes are as asymmetrical as your two feet.
Sphere (SPH): The Main Number
This is the big one. Sphere tells you how much correction you need for basic focus. It is measured in dioptres and always has a plus or minus sign.
- Minus (-) means you are nearsighted (myopic). You see things up close fine but distance is blurry. The bigger the minus number, the blurrier distance is without glasses. -1.00 is mild. -3.00 is moderate. -6.00 and beyond is strong.
- Plus (+) means you are farsighted (hyperopic). Distance might be okay but close-up is harder. In younger people, your eye can often compensate for mild farsightedness, so you might not need glasses until it gets worse or you get older.
If your sphere is "plano" or "PL" or "0.00" that means no sphere correction is needed for that eye. You might still need glasses for astigmatism or reading, though.
Cylinder (CYL) and Axis: The Astigmatism Part
If you have astigmatism, your eye is shaped more like a rugby ball than a basketball. Light focuses at two different points instead of one, making things blurry or slightly doubled at all distances.
Cylinder tells the lab how much astigmatism correction to grind into the lens. Like sphere, it is in dioptres and can be plus or minus (depending on which notation your doctor uses, but it means the same thing either way). -0.50 is very mild. -2.00 is moderate. Above -3.00 is significant.
Axis is a number from 1 to 180 that tells the lab the angle of your astigmatism. Think of it like a clock position. It does not tell you anything useful about your vision; it just tells the lab which direction to orient the correction. If you have cylinder, you will always have axis. If cylinder is blank, axis will be blank too.
About half of all people have enough astigmatism to need correction. It is completely normal.
ADD Power: The Reading Part
If you are over about 40, you probably have an ADD (addition) value on your prescription. This is extra magnification for reading and close-up work. It is always a plus number and it is the same for both eyes (almost always).
ADD does not mean your eyes are getting worse in a disease sense. It means the lens inside your eye is getting stiffer with age and cannot flex to focus up close as easily. This happens to literally everyone. It is called presbyopia and it is why you have probably started holding your phone farther away.
Typical ADD values:
- +1.00 to +1.50: Early presbyopia. You notice some strain with small print.
- +1.75 to +2.25: Moderate. Reading without glasses is getting difficult.
- +2.50 to +3.00: You definitely need reading glasses or progressives.
PD: Pupillary Distance
This is the distance between the centres of your two pupils, measured in millimetres. It is critical for making your glasses because the optical centre of each lens needs to align with your pupil. Average PD is about 62mm for adults, but it ranges from about 54mm to 74mm.
In Canada, your optometrist is required to give you your prescription, including PD, if you ask for it. Some clinics include it automatically. Some make you ask. If it is not on your prescription, call and request it. You need this number if you want to order glasses anywhere.
Prism: The Uncommon One
Most prescriptions do not include prism. If yours does, it means your eyes do not point at exactly the same spot and the lens needs to subtly redirect light to prevent double vision. It is measured in prism dioptres with a direction (base up, base down, base in, base out). If you see prism on your prescription, your glasses need to be made by someone experienced. Do not order prism lenses online unless you really know what you are doing.
An Example Prescription Decoded
Here is a typical prescription and what it means:
OD: -2.25 -0.75 x 180 ADD +2.00
OS: -1.75 -0.50 x 010 ADD +2.00
PD: 63
Translation: Both eyes are mildly nearsighted (right eye a bit more than left). Both have mild astigmatism at different angles. This person is over 40 and needs reading help (+2.00 ADD). Their pupils are 63mm apart. This is a completely ordinary prescription that any lab can make easily.
Is My Prescription "Bad"?
People always ask this. There is no bad prescription, just stronger or weaker ones. A -1.00 might not even need glasses for daily life. A -8.00 means you really cannot function without correction. Neither is a health problem in itself. What matters is whether your eyes are healthy, and that is what the rest of the eye exam checks.
Your Prescription Is Yours
In Alberta, your optometrist must release your prescription to you after an exam. You are not obligated to buy glasses from the same clinic that examined you. Shop around if you want. Just make sure wherever you go has the expertise to measure fitting parameters (segment heights for progressives, for example) and can verify the finished product is made correctly.