On a regular clinic day, I write out 20-30 eyeglasses prescriptions. I’ve been doing this for 25 years now, so it occurred to me that while it’s all second nature to me, the numbers I write down on my eyeglass prescription pad might confuse the person I’m handing it to.
So, let me clear up a few terms and explain why we eye doctors write things the way we do.
OD vs. OS
Understanding what “OD” & “OS” means is the first step to understanding your eyeglass prescription. “OD” stands for oculus dexter while “OS” stands for oculus sinister, which are the Latin terms for “right eye” and “left eye,” respectively.
On an eyeglass prescription, you will typically see the right eye, or “OD”, written first followed by the left eye, or “OS”. However, some prescriptions may have a column titled “OU, “ representing the Latin term oculus uterque, or “both eyes”.
I would agree with you if you said, “Why don’t they just put right and left? What’s the big deal?” It’s because when the optometrist/ophthalmologist faces a patient, he sees the right eye on his or her left side and the left eye on his or her right.
By writing the right eye prescription first, it avoids mixing up right and left eyeglass prescriptions. It’s been written this way for so long that it’s now become a tradition and would be hard to change.
The term “sphere” (SPH) represents the amount of power, measured in diopters (D), needed to correct your vision. The higher the number, the blurrier your vision is when trying to see far away. The (-) minus sign in front of the number means you are nearsighted (you can see up-close but not far away with eyeglasses) or myopic. Vice versa, if there is a (+) plus sign in front of the number, you are farsighted or hyperopic.
Furthermore, the term “sphere” (SPH) represents the idea that the correction for nearsightedness or farsightedness is “spherical” and equal in all centers of the eye.
The “cyl”, or cylinder, refers to the amount of lens power required for astigmatism correction and is represented by the (-) minus sign. The value will be between +/-0.25 and +/-4.00. If you don’t see this designation after the “sphere”, then there is no astigmatism correction, and the patient only is nearsighted or farsighted.
Cylinder means that the added lens power to correct your astigmatism is not spherical; rather, it’s shaped so that one meridian has no added curvature. The meridian perpendicular to the powerless median contains the maximum power and lens curvature to correct your astigmatism.
The axis is where the astigmatism correction is being placed, measured in 1 to 180 degrees. 90 degrees represents the vertical position, and 180 degrees represents the horizontal position. In other words, the axis number lets the lab technician know which direction they should position a cylindrical power in each of your lenses.
Since the axis uses numbers only to describe the position of astigmatisms, you will only see an axis number if there is a cylinder number present.
The ADD is the added magnifying power applied at the bottom of a progressive or multifocal lens to assist with presbyopia. It is represented in (+) and typically ranges from +.075 to +4. It is the power needed to correct the near vision to its sharpest, so you can see the smallest print. If you wear reading glasses, this is usually the power in the bottom part of your bifocals.
PD stands for “pupillary distance” and represents the distance between the center of each pupil (the center of the dark part of the eye). The PD is used to align the center of the lens with the center of the eye. If done incorrectly, it could cause headaches and distorted vision since you are focusing off-center to where you should be. If you need to find your PD yourself, you can download the ReadingGlasses.com PD ruler.
Example Eyeglass Presciption
All of the numbers and abbreviations on an eyeglass prescription can be too confusing. Below you will find a sample prescription to help bring it all together:
RT O.D. -2.00 SPH +2.25 add .5 p.d BD
LT O.S. -2.00 -1.0 x 180 +2.25 add .5 p.d BU
In the above prescription, the right eye (O.D.) requires that your right lens have a -2.00 D sphere for myopia correction. Astigmatism correction is not needed for this eye, so no cylinder or axis power is indicated on the prescription. SPH was added to note that the right eye is only being prescribed a spherical power which is not always the case.
The left eye (O.S.) is being prescribed a -2.00 sphere for myopia as well. The lens will also require a -1.00 D cylinder for correcting an astigmatism present in the left eye. The cylindrical power has its axis on the 180 meridians, which means that the horizontal meridian of the eye will have no additional power for astigmatism while the vertical meridian receives the -1.00 D.
Both eyes require a +2.25 ADD to correct presbyopia (this helps you see fine print up close). Additionally, both eyes are being prescribed a 0.5 prism diopter in each eye - the right eye is base down (bd), and the left eye is base up (bu).
Lastly, the pupillary distance (PD) (distance between the pupils) in the glasses must be set at 55mm.
RX Acronyms and Their Meanings
Below you will find a list of standard optical abbreviations and their meanings in alphabetical order:
ADD: ADDition; additional correction needed for correcting presbyopia.
AR: Anti-Reflective; A type of lens coating that reduces reflections and glare.
AXIS: The angle of the cylinder (CYL) when places on a lens to correct an astigmatism.
BAL: When the Rx requires balance lenses, or the same prescription for both lenses.
BD, BI, BO, BU: The “B” stands for “base” and can mean “base down” (BD), “base up” (BU), “base out” (BO), or “base in” (BI) for prism correction.
CYL: Cylinder; combined with the AXIS to correct an astigmatism.
DS: Diopter Simplex; typically written on an Rx in the CYL field if no astigmatism is present.
DV: Distance vision.
FT: Flat-top bifocal.
NV: Near vision.
OD: Latin for Oculus Dexter and represent the right eye.
OS: Latin for Oculus Sinister and represent the left eye.
OU: Latin for Oculus Uterque and represent the left eye.
PAL: Progressive addition (or adjustment) lens.
PD: Pupillary distance; the measurement of the distance in millimeters between your pupils.
PL: Plano; Latin for “flat”. It means no correction is needed in a section of the lens.
SPH: Sphere; first section on a prescription and represents the correction needed for nearsighted or farsighted vision.
UV: Ultraviolet light; the light that is invisible to the human eye. It is the light that can cause sunburns and eye damage.
VA: Visual acuity; meaning how well the eye can see. This is where you would typically see something like 20/20 vision.
So, there it is. Now you know a little bit about what your prescription numbers mean and how to interpret them correctly.
If you want to learn more, it’s always good to ask your eye doctor any questions you might have concerning your eyeglass prescription.