Prescription for Contacts Vs. Glasses: Are They the Same?

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Prescription for Contacts Vs. Glasses: Are They the Same?

If you have a refractive error and want to wear both contacts and eyeglasses or want to switch from contact lenses to eyeglasses or vice versa, you may be wondering if you’ll need another prescription. We’ve answered this and other related questions below. But first, let’s define prescriptions.

Eye Prescription Definition

An eye prescription is simply a document issued by an Optometrist (or other eye care professionals like ophthalmologists) detailing the specific lens parameters required for visual correction. An eye prescription will contain information/reading discussed below.

Eye Prescription Reading

As per the AAO, an eyeglass prescription will have the following information1:

1. Oculus Dexter (OD)

OD refers to your right eye.

2. Oculus Sinister (OS)

OS refers to your left eye.

3. Oculus Uterque (OU)

OU refers to both eyes.

4. Sphere (SPH)

This refers to the degree of refractive error, meaning the extent of a person’s nearsightedness or farsightedness. SPH indicates the lens power required to fix a patient’s underlying eye condition.

The SPH value/reading may have a minus sign (-) or (+) sign next to the number. The (-) minus sign means you are nearsighted: you see better up close, thus you need distance correction. The (+) plus sign indicates that a patient is farsighted: they see better far away, thus they require near vision correction. If there’s a zero, it means the patient doesn’t need any vision correction.

You may see a reading like -8.00 D. This reading means you have 8 diopters of nearsightedness. The “D” stands for diopters (a measure of the lens power).

5. Cylinder (CYL)

This value is present if the patient has astigmatism (a type of refractive error characterized by an irregularly shaped cornea or lens i.e., an oval as opposed to a round shape). Astigmatism causes blurry vision at all distances. If you have astigmatism, your eyeglass prescription will have a CYL number that corrects this curvature.

6. AXIS

If you have astigmatism, you’ll also see a number (AXIS) ranging between 1 and 180. This number indicates the orientation of your astigmatism i.e., where it is on your cornea.

7. ADD (Addition)

If you are supposed to get certain types of lenses, such as bifocal or progressive lenses, your prescription will include ADD which indicates the power added to the bottom part of the lens (for near vision).

8. Pupillary Distance (PD)

Your prescription will also have PD which indicates the distance between your eyes (measured from the centers of either pupil). PD is usually indicated in millimeters. The reading ensures your glasses fit perfectly.

9. Prism

If you have some eye conditions like double vision, your prescription will also include prim –  a correction built into the lens.

Considering prescriptions for glasses can change due to age and underlying condition, you need to see an optometrist first before getting/changing glasses2. An eye doctor will ensure all the above information in your prescription is up-to-date.

Are Contact and Glasses Prescription the Same?

No! They are two separate prescriptions. While both contact eyeglasses prescriptions and eyeglasses prescriptions are meant to solve the same underlying eye condition, they differ because of several factors. 

First and foremost, eyeglasses sit further away from the eye (about 12mm away) while contacts rest directly on the eye’s surface. This difference in distance requires a different corrective power for each type of lens. The key differences are captured in the prescription.

Contact Prescription Vs Glasses

Generally, eyeglass prescriptions only include the SPH, CYL, and Prism (sometimes). A prescription for contact lenses will be more detailed with additional specifications such as:

• Base curve (BC): A measure of the curvature of the lens to ensure a perfect fit
  
• Diameter (DIA): A measure of the size of the contact lens in relation to the size of your eyes.
  
• Lens material and brand: Contact lens prescriptions also capture the type of contact lens materials used to make the lens as well as the brand or manufacturer.
  

Can You Convert Glasses Prescription to Contacts?

As mentioned, you can’t directly convert an eyeglasses prescription to a contact lens prescription since eyeglass prescriptions don’t contain important additional information i.e., base curve and diameter required for effective correction. If you try converting, you could increase your risk of eye infections and worsen your eye condition.

How to Get a Contact Lens Prescription

You’ll need to visit an optometrist to have a specialty contact lenses exam done. The exam will ensure important eye measurements are taken i.e., corneal curvature that ensures you get the best contact lenses that match your eyes and physical activities or lifestyle.

How to Get a Glasses Prescription

You’ll need to visit an optometrist to have a comprehensive eye exam done. The exam should uncover important eye measurements and the underlying refractive error, ensuring an accurate up-to-date prescription. A comprehensive eye exam will also uncover any other eye health issues through different tests to assess peripheral vision, overall field of vision, etc.

How Long Is An Eyeglass Prescription Good for?

Generally, eye prescriptions are valid for a year or two. This is simply because the eyes can change over time due to several factors (age-related changes that affect vision). What’s more, an underlying eye condition can affect vision and produce changes. 

Some medications can also affect vision. For these reasons, it’s important to go for regular eye exams as soon as required: if you notice vision changes or when your current prescription glasses aren’t effective.

Types of Prescriptions

Prescriptions can be categorized based on the underlying condition they treat or factors like prescription power. Here are the main types:

a. Eyeglass Prescriptions

The most common types of eyeglass prescriptions include:

• Single-vision prescription: This prescription is for single-vision glasses with one prescription power throughout. This type of prescription is issued for nearsightedness,  farsightedness, and astigmatism correction.
  
• Multifocal lens prescription: This prescription is for lenses with multiple prescription powers allowing clear vision at different distances: bifocal, trifocal, and progressive lens prescriptions.
  

b. Contact Prescriptions

• Sphere: This prescription corrects farsightedness and nearsightedness.
  
• Cylinder: This prescription corrects astigmatism.
  
• Sphere & Cylinder: This prescription corrects farsightedness, nearsightedness, and astigmatism.
  

Getting the Right Prescription for Your Needs

If you are looking to update your look with a new pair of prescription eyeglasses, or want to experience the freedom of contact lenses, it is crucial to start with the right medical advice.

Whether you need soft contacts, daily disposables, multifocals, or silicone hydrogel lenses, a valid prescription is the first step. Attempting to use a glasses prescription for contacts (or vice versa) can lead to eye strain, poor vision, and potential health complications.

Besides getting a valid prescription, a visit to your local eye doctor allows you to ask questions about specialized care, from myopia control and vision therapy to laser eye surgery options.

Reach out to a certified North York Optometrist in your area today to schedule a comprehensive eye exam or a specialty contact lens fitting. Your eyes deserve the best care possible.

References

[1] American Academy of Ophthalmology: https://www.aao.org/eye-health/glasses-contacts/how-to-read-eyeglasses-prescription#:~:text=What%20does%20%E2%80%9Csphere%20(SPH),the%20stronger%20your%20prescription%20is

[2] Cleveland Clinic: https://health.clevelandclinic.org/how-to-read-your-eye-prescription