Defining Eye Prescriptions: A Guide to Understanding Your Vision Prescription

Eye prescriptions are simply instructions that eye care professionals (optometrists) offer for corrective lenses (eyeglasses or contacts). They specify all the information required to address the underlying vision condition, whether that be myopia (nearsightedness), hyperopia (farsightedness), astigmatism, presbyopia, etc.¹

Eye prescriptions are usually given after a person has undergone an eye exam; the results of an eye exam explain the information that will be contained in the prescription. So, how do you read an eye prescription? Discover this and more below.

How to Read an Eyeglasses Prescription?

Optical prescription (whether for contact lenses or eyeglasses) contains multiple values that precisely correct a person’s vision. To understand the values contained in prescriptions, it’s important to understand refractive errors and how different errors affect vision. For instance:

Myopia makes it difficult to see distant objects
Hyperopia makes it difficult to see close objects
Presbyopia makes it difficult to focus on close objects (as you age)
Astigmatism causes blurred vision because of an irregularly shaped cornea

After a comprehensive eye exam, each of these conditions will result in a prescription with different key measurement values. A combination of these different values results in a custom lens that can offer optimal vision clarity. The values that you are most likely to see in your eye prescription include, but aren’t limited to:

Sphere (SPH)
Cylinder (CYL)
Axis
ADD (addition)
Prism

You may also see abbreviations like OD (right eye) or OS (left eye). Let’s discuss each of these values and what they mean.

Defining Eye Prescriptions_ A Guide to Understanding Your Vision Prescription - Blog, Kodak Lens Vision Centres

What is OD vs. OS in an Eye Prescription?

OD (Oculus Dexter) and OS (Oculus Sinister) refer to measurements/values for the right and the left eye respectively.

Oculus Dexter is Latin for the right eye. If the prescription has OD, any measurements underneath or beside are for your right eye.
Oculus Sinister is Latin for the left eye. If the prescription has OD, any measurements underneath or beside are for your left eye.

The distinctions are necessary because, in many cases, each eye may have different vision correction needs. Indicating OD or OS ensures each eye gets the correct lens power. Let’s now focus on other key values/measurements in eye prescriptions.

Understanding Key Terms in an Eye Prescription

Let’s take a closer look at some crucial terms you might find on your eye prescription, which will make it easier to understand the important information compiled there.

i. What Does SPH Mean?

SPH (Sphere) refers to the lens power required to fix an underlying vision condition (refractive error like myopia).

Lens power is indicated in diopters so the SPH field can be written as -9.00 D or +9.00D.

If a (-) minus sign is next to SPH, this means that you have nearsightedness, meaning that you see nearby objects better, so you need distance correction. -9.00 D in the SPH field would therefore mean you have 9 diopters of myopia (nearsightedness).
If a (+) plus sign is next to SPH, this means you have farsightedness, meaning that you see objects better when they are far away so you need near vision correction. +9.00 D in the SPH field would therefore mean you have 9 diopters of farsightedness.
For a 0 value, there would be no correction needed. Generally, the further away a value is from zero (whether negative or positive) means a stronger prescription; the appropriate lens power is needed to correct.²

What is CYL on an Eye Prescription?

If you have a specific refractive error, like astigmatism, your prescription will show a cylinder number (CYL). Since astigmatism is characterized by cornea curvature (oval-shaped or egg-shaped cornea instead of being round like a basketball), the CYL number indicates the lens power in diopters (usually negative) required for astigmatism correction.³

What is “Axis” in an Eye Prescription?

If you have astigmatism, your prescription will also have an axis number indicated in degrees (1 to 180 degrees). This number tells the precise location of the astigmatism on the cornea i.e., the angle or orientation of the astigmatism.

Why is Axis Important?

The axis is critical because it guides the precise placement of cylindrical power in the lenses to correct vision.⁴

What Does Prism Mean?

If you have eye alignment conditions like diplopia (double vision), strabismus (cross eyes), or a binocular vision condition, your prescription will have “prism” – the correction required to align the eyes correctly.

The prismatic power (indicated in diopters) is usually indicated alongside a base direction i.e., base up/down/in/out specifying how the prism correction needs to be oriented in a lens.

Prism lenses correct eye muscle imbalances by bending light before it enters the eyes. This helps to realign images seen by the eyes. Prism lenses help the brain fuse two images into one clear image.⁵

What Does “ADD” Mean?

If you have conditions like presbyopia that make it hard to focus on near objects as you age, your prescription will include “ADD”, which indicates the ADD power or magnifying power required for proper near vision.

The ADD value is simply the additional magnifying power required for near vision. It is common in multifocal lenses like reading glasses, progressive lenses, bifocals and trifocals.⁶

The importance of PD measurement for Eyeglasses

In order for your eye prescription to correct your vision, Eyeglasses also require a PD value which indicates interpupillary distance, meaning the distance in mm between both eyes of the center light reflex in the pupils. A correct PD is critical for ensuring you’re looking through the correct part of your lenses for optimal vision and comfort.

Without a PD value, there wouldn’t be a precise way of ensuring your eyeglasses are perfectly aligned with your eyes, eliminating eye fatigue, eye strain, and blurry vision.⁷

What is “OC” and “SEG” Height?

If you suffer from eye strain or poor vision in your lenses, the OC height (optical center height) measurement will help. If you have presbyopia (age-related blurred near vision) and you’re prescribed multifocal lenses, the SEG Height (segment height) measurement will ensure proper lens placement so you can best use the multifocal lens design.

Optical center height is an indication of the vertical distance from the center of your pupils to the bottom of the lens. The optical center is the precise point on a lens where light passes without being bent. Without an accurate OC height measurement, the lens’s optical center wouldn’t be able to align with your pupil.
SEG Height is an important measurement for bifocals, trifocals and progressive lenses. The measurement indicates the vertical distance (from the bottom of a lens to the top line of the lens bifocal segment or the start of the progressive). The SEG height measurement simply tells the lens manufacturer precisely where they should place the distance, intermediate and near vision parts for your unique eye prescription.

Both OC and SEG height measurements are critical for personalized lens fitting that ensures optimal vision comfort and clarity.

Types of Eyeglass Prescriptions

Having looked at the measurements/values in eye prescriptions, let’s discuss the four common types of eyeglass prescriptions:

Single-Vision Lenses

Single-vision lenses are the most common type of prescription for eyeglasses. As the name suggests, single-vision lenses have a single power of correction for the entire lens surface. They are prescribed to correct a single vision problem such as myopia, hyperopia, or astigmatism.

Bifocals and Trifocals

As the name suggests, bifocals contain two distinct power zones on the lens for correcting vision at two different distances: one for near vision (reading) and the other for distance vision. Trifocal lenses are multifocal lenses that correct vision at three different zones: near, distance, and intermediate.

Progressive Lenses

Progressive lenses offer a seamless transition between near, distance, and intermediate vision. They eliminate visible lines in bifocals offering a more visually appealing look. The lenses are prescribed for presbyopia and are the most recommended lenses to treat it.

Prism Lenses

Prism lenses are mainly used to correct eye alignment issues like double vision (diplopia) or binocular vision issues. The lenses bend light and realign images seen by each individual eye. Lately with new prism technology, there have been specialty cases where prism can help relieve post-concussion, migraine, equilibrium and many other unique symptoms.

How Long Are Eye Prescriptions Good for?

Generally, the validity of eye prescriptions tends to vary based on factors like type, age, eye health, and existing regulations.

Eyeglass prescriptions: Typically, a prescription for eyeglasses is usually valid for one or two years. However, this can vary based on other factors discussed below.
Contact lens prescriptions: Generally, contacts have a shorter validity period (usually one year) since contacts “sit” directly on the eye, making constant monitoring a priority. Annual eye exams are therefore recommended.
Age: The age of a patient will also dictate eye prescription validity. The validity of children’s prescriptions is usually shorter because they experience more vision changes than adults.
Eye health: Some eye conditions like high prescriptions may demand more/frequent eye exams, resulting in shorter prescription validity.
Regulations: Some jurisdictions may have maximum/minimum prescription validity periods. Learn more here.

In a nutshell, you should adhere to the prescription expiry date given by your Optometrist/eye care specialist. Most importantly, go for regular eye exams as required to ensure you have an updated nearsighted, multifocal, astigmatism, and/or farsighted prescription at all times. You should also visit your Optometrist immediately if you experience any eye health issues.

How to Get Your Prescription and What to Do Next?

If your current glasses or contacts aren’t effective and you suspect you have the wrong eye prescription, you should schedule an eye exam to confirm.

If you reside in Toronto or the Greater Toronto Area, you can visit leading Ontario-based Optometrists at Kodak Lens Vision Centres for comprehensive eye exams that provide accurate eye prescriptions using the most advanced eye care technology. Visit any of the seven locations: Ajax, North York, Etobicoke, Rexdale, Toronto, Scarborough or Riverdale.

Order New Eye Glasses or Contact Lenses in Toronto or the Greater Toronto Area

After receiving an updated eye prescription, you can head over to the Kodak Lens Vision Centres for eyeglasses with single-vision or multifocal lenses. Check out their online store and start shopping for contact lenses, eye care products, eye drops and more!

Ready to Get Your Updated Eye Prescription?

You need the correct eye prescription to be able to correct your underlying vision condition. As mentioned, the prescription you use will also determine important factors like eye comfort. To ensure you get the correct updated prescription, you must go for a comprehensive eye exam first.

Kodak Lens Vision Centres offer regular eye exams ensuring patients have the latest information they need about their eye health. Book an appointment now! You can also purchase eyeglasses, contact lenses, and prescription sunglasses to address any underlying visual needs.

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