You walk out of an eye exam with a small slip of paper covered in abbreviations, numbers, and plus or minus signs. Most patients fold it into their wallet without understanding a single line of it. You are paying for this information and relying on it for the glasses or contact lenses that you will wear every day. You should understand what it means.
This guide translates every number and abbreviation on a standard eyeglass prescription into plain language.
The Basic Layout
A standard prescription has separate entries for the right eye and the left eye. The right eye is listed as OD, from the Latin oculus dexter. The left eye is OS, from oculus sinister. You will sometimes see OU, which means both eyes together. The right eye is always listed first. If you see a prescription with two rows of numbers, the top row is your right eye and the bottom row is your left.
SPH: Sphere
The sphere value indicates the overall focusing power needed to correct your vision. It is measured in diopters. A minus sign in front of the sphere number indicates myopia, or nearsightedness. Your eye is focusing light in front of the retina rather than on it. The further the number is from zero, the stronger the prescription. A minus three is more myopic than a minus one.
A plus sign indicates hyperopia, or farsightedness. Your eye is focusing light behind the retina. Plus sphere also appears in the near correction of progressive lens prescriptions for presbyopia, where it represents the additional power needed to see clearly up close. If the sphere box shows plano or 0.00, no sphere correction is needed in that eye.
CYL and Axis: Cylinder and Axis
These two values always go together and both relate to astigmatism. Astigmatism occurs when the cornea or the lens inside the eye is not perfectly spherical, causing light to focus at two different points rather than one. The result is blur or distortion at some or all distances.
CYL is the amount of astigmatism correction needed, expressed in diopters. It comes with a minus or plus sign depending on the convention your optometrist uses. If the CYL box is blank or shows 0.00, you have no clinically significant astigmatism. Axis is a number between 1 and 180 that indicates the orientation of the astigmatism correction. It tells the lens manufacturer which direction to position the cylindrical power in the lens. The axis number alone means nothing without the cylinder value alongside it.
ADD: Addition Power
The ADD value appears on prescriptions for patients who wear reading glasses, bifocals, or progressive lenses. It is always a plus power and represents the additional magnification needed for near tasks on top of the distance correction. ADD values typically range from plus 0.75 in early presbyopia to plus 3.50 in more advanced cases. The ADD value increases gradually as presbyopia progresses with age and is the same in both eyes in the vast majority of prescriptions.
PD: Pupillary Distance
PD is the measurement in millimetres of the distance between the centres of your pupils. It is what ensures the optical centres of the lenses in your glasses align precisely with your eyes. When the PD is even slightly off, the optical performance of otherwise correct lenses is compromised, causing distortion and sometimes headaches or eye strain with new glasses. Your PD may be recorded as a single binocular measurement or as two separate monocular measurements, one for each eye from the bridge of the nose.
Prism and Base
Prism correction appears on prescriptions for patients with binocular vision problems where the eyes have difficulty aligning on the same point. Prism redirects light to compensate for the alignment error. The base value indicates which direction the prism is oriented. The majority of standard prescriptions do not include prism.
Why Your Contact Lens Prescription Is Different
A contact lens prescription contains all the same core refractive information but adds details specific to contact lenses: the base curve of the lens, which must match the curvature of your cornea, the diameter of the lens, and an approved brand name. These additions are why a glasses prescription cannot simply be used to order contact lenses. A specific contact lens exam in Edmonton is required to obtain a valid contact lens prescription, even if you have a current and accurate glasses prescription.
How Long Is a Prescription Valid?
In Alberta, eyeglass prescriptions are typically valid for one to two years, and contact lens prescriptions for one year, depending on the optometrist’s clinical judgement. If your prescription is over a year old, a comprehensive eye exam in Edmonton before ordering new glasses or lenses ensures you are working from accurate measurements rather than a prescription that may no longer reflect your current vision.
Questions Edmonton Patients Ask About Reading Their Prescriptions
Q: My sphere number got more negative compared to last year. Does that mean my eyesight got worse?
If you are myopic (have a minus sphere), a more negative number means your prescription has increased and your distance vision has changed. This is common during the teens and twenties when myopia is still developing. In adults, prescription changes tend to stabilize, though minor fluctuations are normal. Significant changes in a short period are worth discussing with your optometrist, particularly for children and teenagers, where myopia management options may be appropriate.
Q: What does it mean if my two eyes have very different sphere numbers?
A significant difference in prescription between the two eyes is called anisometropia. It is not uncommon and does not mean something is wrong, but it does mean the lenses you need will have noticeably different thicknesses. Higher-index lens materials reduce this thickness difference. In children, a significant prescription difference between the two eyes is a risk factor for amblyopia and is one reason why annual pediatric eye exams matter even when a child shows no obvious symptoms.
Q: My prescription shows a cylinder and axis. What does astigmatism actually feel like?
Astigmatism typically produces blur or distortion at one or more distances. Some people describe it as lights appearing to streak or smear rather than appear as a clean point. Others experience general blurring or difficulty reading fine text. Many people with mild astigmatism are unaware of it until the correction is applied, at which point the improvement is immediately obvious.
Q: Can I use my glasses prescription to buy glasses online?
You can, but there are practical considerations. Online glasses work best for simple prescriptions without significant cylinder, high sphere powers, or progressive lens requirements. Your PD measurement needs to be accurate. Strong prescriptions, progressive lenses, and prescriptions with significant astigmatism are areas where in-person fitting produces meaningfully better results. Read our guide on affordable glasses in Edmonton for a balanced look at when online works and when it does not.
Book Your Next Eye Exam in Edmonton
If you have questions about your prescription, it has been more than a year since your last exam, or you want to update your glasses or contact lenses, contact Eyes On Optometry to book an appointment in Edmonton. Our team is happy to walk you through your results and explain what each number means for your vision and your daily life.