WHAT DOES MY PRESCRIPTION MEAN?
DECODING YOUR PRESCRIPTION — BECAUSE YOUR EYE DOCTOR ISN’T ALWAYS AROUND TO EXPLAIN IT
That slip of paper your eye doctor hands you after an exam looks like a random collection of numbers, abbreviations, and symbols. It isn’t. Every single character on that prescription is critical information that determines how your lenses are made — and getting any of it wrong means lenses that don’t work, eyewear that fails you, and vision that falls short when you need it most.
Here’s what it all actually means.
OD and OS — Which Eye Is Which
OD (Oculus Dexter) = your right eye. OU (Oculus Sinister) = your left eye. OU = both eyes together.
Latin. Because apparently eye care has been around long enough to keep the ancient terminology. What matters is that each eye gets its own set of numbers — because your right eye and left eye almost never need identical correction.
Sphere (SPH) — How Strong Your Lenses Need to Be
This is the big one. Sphere measures the degree of nearsightedness or farsightedness in your prescription.
A minus (–) number means you’re nearsighted — you see clearly up close but struggle at distance. A plus (+) number means you’re farsighted — distance is fine but close-up work gives you trouble.
The further from zero — in either direction — the stronger your prescription. Simple. Powerful. Non-negotiable.
Cylinder (CYL) — Correcting Your Astigmatism
If you have a CYL number on your prescription, you have astigmatism — meaning your cornea isn’t perfectly round. Don’t panic. It’s extremely common. The cylinder number measures the degree of correction needed to compensate for that irregularity.
Like sphere, it uses plus or minus values. No CYL number on your prescription? You either have no astigmatism or it’s so minor it doesn’t require correction.
Axis — Where the Astigmatism Lives
The axis works hand-in-hand with cylinder. It’s a number between 1 and 180 that tells the lens maker the exact orientation of your astigmatism correction. Cylinder without axis is useless. Axis without cylinder is meaningless. They’re a package deal — and both need to be exactly right.
ADD — For the Reading Boost
ADD (Addition Power) shows up on prescriptions for people who need different correction for near and distance vision — typically showing up in bifocal or progressive lenses. It’s always a plus power and represents the additional magnification needed for reading or close-up work.
If you’re over 40 and finding yourself holding your phone at arm’s length to read it — ADD is likely about to become your new best friend.
Prism — The Rare One
Not everyone has this. In fact, most don’t. Prism correction addresses eye alignment issues — when your eyes don’t naturally work together the way they should. It’s measured in prism diopters and comes with a base direction (Base In, Base Out, Base Up, Base Down) that tells the lab exactly how to orient the correction.
If you have prism on your prescription, you already know your vision situation is specific. Getting it right in safety eyewear matters even more.
Pupillary Distance (PD) — The Number Labs Can’t Work Without
PD measures the distance between the centers of your pupils — typically somewhere between 54mm and 74mm for adults. It tells the lab exactly where to center your optical correction in each lens.
Get the PD wrong and even a perfect prescription produces lenses that strain your eyes, cause headaches, and deliver blurred or distorted vision. It’s one of the most critical measurements in the entire process — and one of the most commonly overlooked.
At Z87 OPTICS, we never overlook it.
The Bottom Line
Your prescription isn’t just a formality. It’s the blueprint for lenses that let you see clearly, work safely, and perform at your best — every single shift, every single day.
Understanding it puts you in control. And at Z87 OPTICS, we make sure every number on that prescription is handled with the precision it deserves.