Your Contact Lens Prescription
Eye care practitioners use a phoropter to measure your vision very precisely, so they can prescribe contact lenses that will give you the best vision possible.
People sometimes ask, "Is my contact lens prescription the same as my glasses prescription?"
The answer is often not obvious, although in the great majority of cases, they are not the same.
Let's take an example of an eyeglass prescription:
- OD (right eye): -4.50 -1.50 x 180
- OS (left eye): -3.25 DS
The amount of correction is expressed in diopters, sometimes abbreviated as "D." Here, the left eye needs a correction of -3.25 diopters. "DS" (diopters sphere) indicates a spherical correction only, and no astigmatism correction. The minus (-) sign indicates correction for nearsightedness (myopia); a plus (+) sign would mean farsighted correction (hyperopia).
Here, the right eye has a spherical correction of -4.50D and an astigmatism correction of -1.50D. The degree axis of the astigmatism correction is 180.
How Contact Lens Prescriptions Differ
Since eyeglasses are usually about 12 millimeters from the eye, the prescriptive power of a contact lens — which is positioned directly on the tear film of the eye — will be slightly less nearsighted. This becomes significantly different with prescriptions of 4D or greater. Therefore, in our example, the power of a contact lens will be less than the eyeglass prescription for the right eye.
A GP lens automatically corrects the astigmatism of the surface of the cornea by molding it into a spherical shape. So a GP lens power will typically be close to the spherical correction of the glasses prescription.
However, this will depend on the curvature of the back surface of the contact lens and the amount of spherical correction for myopia or hyperopia. For example, in the left eye:
- If a GP lens is selected with a base curve radius that's one-half diopter flatter (less curved) than the cornea, a contact lens power of -2.75D would be predicted.
- If the base curve radius is a half diopter steeper (more curved) than the cornea, a contact lens power of -3.75D would be predicted.
Soft Lens vs. GP Contact Lens Prescriptions
A spherical soft lens does not correct for astigmatism. For someone wearing soft lenses, a toric soft lens would be needed if there is, at minimum, 0.75D of astigmatic correction. Therefore, in our example, the right eye would require either a GP lens or a soft toric lens.
Because the spherical correction for the right eye is 4D or greater, and because toric soft lenses are typically available in 0.25 diopter increments, the contact lens prescription may be closer to -4.25 - 1.25 x 180. The exact prescription can be determined only with diagnostic lens fitting by an eye care professional.
For the left eye, it's possible that a spherical soft lens with a correction identical to the glasses prescription (-3.25D) will work, but again, this determination is best made after a diagnostic fitting.
Can I Obtain My Contact Lens Prescription?
You are entitled to a copy of your contact lens prescription once it has been finalized. The point at which your Rx is final is the decision of your eye care practitioner.
In some cases this can occur after the diagnostic fitting. In many cases — notably with GP lenses and specialty designs such as bifocal, toric, keratoconus, or post-surgical — it may take several visits before a successful fit has been achieved and a prescription can be provided.
If the lenses will not be dispensed by the prescribing practitioner, it is important that whoever fills the prescription does so completely and accurately. A soft lens Rx typically includes a lens material name, design, power, base curve, and diameter. GP lens prescriptions usually have this information and more, such as the peripheral curve radii, center thickness, a special peripheral design and an optical zone diameter.
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[Page updated February 2013]
