Contact lenses and glasses are two of the most popular forms of vision correction. While they both serve the same purpose of correcting refractive errors, they use different technology and are worn in different positions relative to the eye.
More specifically, contact lenses sit directly on the eye’s surface, while glasses sit about 12 millimeters away from the eyes. This difference in proximity to the eye means that the strength of the lenses has to be adjusted accordingly, resulting in distinct prescriptions depending on whether you’re wearing glasses or contacts.
Understanding the Vision Prescription
Often, when individuals visit their optometrist for a routine eye exam, they leave with a vision prescription.
The numbers on this prescription include the necessary parameters for vision correction for glasses or contacts. Once you know what these numbers mean, understanding your vision prescription becomes straightforward.
Reading left to right, a typical prescription includes parameters such as a sphere (SPH), a cylinder (CYL), and an axis.
The sphere indicates the lens power necessary to correct myopia (nearsightedness) or hyperopia (farsightedness), and it is measured in diopters (D), which increase in 0.25 increments. A negative sign (-) signifies myopia, while a positive (+) indicates hyperopia.
The next column in the prescription is the cylinder value, which denotes the lens power needed to correct astigmatism and is used to describe irregular cornea or lens curvature. This value is typically negative.
The axis is a number between 0 and 180 degrees that specifies the orientation of astigmatism correction. This represents the angle at which the cylindrical power should be positioned relative to the corrective lens.
A cylinder value will always be accompanied by an axis value. This means that, if you don’t have an astigmatism, there won’t be an axis value on your prescription.
For example, a prescription might read -2.25, -1.50 x 125. The sphere value in this example indicates moderate myopia, with the cylinder and axis values indicating moderate astigmatism.
For those who require correction for presbyopia (a condition that makes it harder for older adults to see things up close), a final “add” value is included at the end of the prescription. This value provides additional strength required for reading or multifocal lenses.
Occasionally, a vision prescription may include prism correction to help with binocular vision concerns like double vision. A prism value is accompanied by a base direction (up, down, or outwards) that outlines its orientation.
The same base parameters (sphere, cylinder, axis, and add) are used in contact lens and eyeglass prescriptions. However, their measurements usually differ slightly because of the distinct way each corrective method interacts with your eyes.
Why are Contact Lens and Eyeglass Prescriptions Different?
Contact lenses and eyeglasses require slightly different prescriptions because of their placement relative to the eye.
Eyeglasses sit about 12 millimeters away from your eyes and rest on the bridge of your nose, while contacts rest directly on the corneal surface. This proximity affects the power needed for ideal vision correction.
For instance, an individual with myopia may require a different lens power for contacts compared to their glasses because the lens’s distance from the eye affects how light focuses on the retina.
Contacts can come in varying sizes. A contact lens prescription also includes specific measurements like base curve and diameter to ensure that the lens’s dimensions fit over the surface of the eye. These measurements aren’t needed for glasses.
That’s why it’s necessary to have distinct prescriptions for each form of vision correction. Substituting one for another is ill-advised.
Wearing a prescription that’s too weak or too strong can lead to eye strain, headaches, or blurry vision. Using prescriptions only for their intended purpose is vital for maintaining good visual clarity, comfort, and ocular health.
How Long Are Prescriptions Valid?
Contact lens prescriptions are typically valid for just one year. They require annual renewal because of how directly the lenses sit on the eye’s surface and the extra care and maintenance that contacts require.
An annual contact lens exam and fitting involves assessing vision changes, examining ocular health, and confirming the shape and fit of the lenses to provide clear and comfortable vision.
In contrast, eyeglass prescriptions are valid for two years because they sit further away from the eye. Nonetheless, routine eye exams are recommended at least every two years to keep your vision up to date and maintain good ocular health.
Which is Better: Contacts or Glasses
Choosing between contacts and glasses is a personal decision influenced by preference, lifestyle, comfort, and ocular health.
Eyeglasses offer a maintenance-free option, as they don’t require cleaning solutions or intricate handling. They can act as fashion accessories, protect your eyes from wind and debris, and don’t come into contact with your eyes, thereby minimizing the risk of irritation or infection.
On the other hand, contact lenses provide a natural field of vision without obstruction from eyeglass frames. Contacts remain unaffected by weather conditions like rain and fog that can affect eyeglass lenses, but they require meticulous care to prevent irritation.
There’s no “right” answer, but consulting with your optometrist to confirm that your choice aligns with your health and vision requirements is always wise.
Whether you opt for the classic appeal of glasses or the flexibility of contact lenses, the goal of these corrective options is to seamlessly integrate into your life, enhancing both your vision and comfort.
Visit Us!
Though similar in function, contact lens and eyeglass prescriptions differ due to their placement and proximity to our eyes. They each serve their own optical needs, requiring precise adjustments for ideal vision correction.
Whether you’re interested in browsing our unique selection of frames or updating your contact lens prescription, connect with our team at Professional Eyecare Centers to schedule a visit!
