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Why Your Contact Lens Prescription Differs from Glasses

A contact lens prescription and a glasses prescription are related but not interchangeable. Contact prescriptions include fitting parameters — base curve and diameter — that glasses prescriptions omit, and the corrective power is sometimes adjusted because contacts sit directly on the eye rather than several millimeters away. Federal law requires your eye care provider to give you your contact lens prescription after the fitting, and prescriptions must remain valid for at least one year.

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What information is in a glasses prescription?

A glasses prescription (also called a spectacle refraction) contains:

  • Sphere (SPH) — the main power correction for nearsightedness (negative number) or farsightedness (positive number)
  • Cylinder (CYL) and Axis — corrections for astigmatism, if present
  • Addition (ADD) — for bifocal or progressive lenses, the reading power added at the bottom of the lens
  • Pupillary distance (PD) — used by opticians to center lenses in frames

This tells an optician everything needed to make glasses. It does not specify how a lens will sit on your eye, which is a separate clinical determination 1.

What extra information does a contact lens prescription include?

A contact lens prescription contains everything in a glasses prescription, plus parameters specific to the physical lens:

  • Base curve (BC) — the curvature of the back surface of the lens, matched to the curvature of your cornea so the lens fits properly and stays centered
  • Diameter (DIA) — the overall size of the lens
  • Brand and material — because fitting parameters vary between manufacturers, a prescription is specific to a particular lens product
  • Power — which may differ slightly from your glasses prescription (see below)

The contact lens prescription also carries an expiration date. Under the federal Contact Lens Rule, prescriptions must remain valid for at least one year unless there is a documented medical reason for a shorter duration 2.

Why is the power sometimes different between contacts and glasses?

Glasses lenses sit several millimeters in front of the cornea, while contact lenses rest directly on the eye's surface. At higher prescriptions (roughly beyond ±4.00 diopters), this difference in distance — called vertex distance — means the effective power reaching the retina differs depending on lens position. Eye care providers apply a mathematical vertex-distance correction when converting spectacle power to contact lens power 1.

At lower prescriptions the difference is clinically negligible and the powers may appear identical. At stronger prescriptions they can differ noticeably, which is one reason ordering contacts based on a glasses prescription can result in the wrong correction.

What happens at a contact lens fitting appointment?

A contact lens fitting is a separate appointment from a routine eye exam, though it is often combined in the same visit. During the fitting:

1. Your eye care provider measures your corneal curvature (keratometry) to select the appropriate base curve 2. A trial lens is placed on your eye and evaluated for fit — movement, centration, and full corneal coverage 3. Your vision is checked with the trial lens in place 4. Adjustments are made and a final lens is selected 5. You receive instruction on insertion, removal, and lens care

For astigmatism, toric lenses add complexity because they must align precisely on the eye. Significant astigmatism or presbyopia may require multiple trial lenses before the right match is found.

After a successful fitting, you receive your final contact lens prescription. Under the Fairness to Contact Lens Consumers Act, your provider must give you a copy at the end of the fitting — even if you do not ask for it and even if you purchase lenses elsewhere 2.

Can I order contacts with my glasses prescription to save time?

No. A valid contact lens prescription — including brand-specific fitting parameters — is legally required to purchase contact lenses in the United States. Using your glasses prescription risks purchasing lenses with incorrect power, an ill-fitting base curve, or a diameter that does not match your corneal geometry. Poorly fitting lenses can cause discomfort, reduced oxygen delivery, and corneal surface damage over time 1.

If your contact lens prescription has expired, you will need an updated fitting appointment before ordering new lenses. An optometrist or ophthalmologist performs contact lens fittings. Gale can help you find the right eye care provider in your area.

Common questions

How long is a contact lens prescription valid?

Under the federal Contact Lens Rule, contact lens prescriptions must remain valid for at least one year unless a prescriber documents a legitimate medical reason for a shorter duration. Annual expiration encourages regular eye health monitoring and ensures the fitting remains appropriate as your eyes change.

Does my eye doctor have to give me my contact lens prescription?

Yes. Under the Fairness to Contact Lens Consumers Act, your eye care provider must give you a copy of your contact lens prescription at the end of the fitting, even if you did not ask for it and even if you plan to purchase lenses elsewhere. No additional fee may be charged for releasing the prescription.

Can I switch brands if my contact lens prescription lists a specific brand?

Not without a new fitting or explicit provider guidance. Different brands have different base curves, diameters, and materials. A lens that fits well in one brand may not fit properly in another with the same power. Switching brands should always involve your eye care provider.

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When to seek care for contact lens problems

  • —Eye pain that does not resolve after removing the lens
  • —Significant redness or discharge
  • —Vision that is not corrected well despite wearing the right lens
  • —A white spot on the cornea (possible infection or ulcer)

Remove the contact lens and contact your eye care provider the same day for any of the above symptoms. A corneal ulcer requires urgent treatment.

This article provides general educational information about contact lens and glasses prescriptions. Lens selection and fitting require in-person evaluation by a licensed eye care professional.

References

  1. 1.Jacobs DS, Afshari NA, Bishop RJ, Keenan JD, Lee J, Shen TT, Vitale S; American Academy of Ophthalmology Preferred Practice Pattern Refractive Management/Intervention Panel (2023). Refractive Errors Preferred Practice Pattern. Ophthalmology. doi:10.1016/j.ophtha.2022.10.031 ✓Contact lens fitting parameters, vertex distance correction, and the clinical distinction between spectacle and contact lens prescriptions
  2. 2.Federal Trade Commission (2020). Contact Lens Rule (16 CFR Part 315): Amendments — Fairness to Contact Lens Consumers Act. Federal Register / FTC Legal Library. link ✓Requirement that prescribers release the contact lens prescription to patients at the end of a fitting at no extra charge; one-year minimum prescription validity unless medically documented otherwise

2 sources, numbered by first appearance. General health information, not medical advice — synthetic demonstration content.