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The Difference Between A Contact Lens And Eyeglass Prescription


Eyeglass vs. Contact Lens Prescription

When it comes to correcting vision, individuals have the option of choosing between eyeglasses and contact lenses. While both serve the same purpose, there are significant differences between the two in terms of prescription requirements and considerations. In this article, we will explore the differences between a contact lens prescription and an eyeglass prescription, highlighting why an eyeglass prescription cannot be used to purchase contact lenses, the necessity of a contact lens eye exam and fitting by an optometrist, and the validity period of a contact lens prescription.

The Basics Of An Eyeglass Prescription

An eyeglass prescription, also known as a spectacle prescription, is a written document provided by an optometrist or ophthalmologist. It specifies the lenses needed to correct a person's vision based on the results of a comprehensive eye examination. Eyeglass prescriptions are primarily designed for lenses that are placed within a frame and worn on the face.

Components Of An Eyeglass Prescription

An eyeglass prescription consists of several key elements that help determine the lens power needed for optimal vision correction. These components include:
  • Sphere (SPH). The sphere measurement indicates the lens power required to correct common refractive errors, such as nearsightedness (myopia) or farsightedness (hyperopia). It is denoted with a plus sign (+) for farsightedness and a minus sign (-) for nearsightedness.
  • Cylinder (CYL). The cylinder measurement is used to correct astigmatism, a condition that results in blurred vision due to an irregularly shaped cornea. It is accompanied by an axis value, which indicates the orientation of the astigmatism.
  • Axis. The axis value is provided when there is astigmatism present and indicates the direction of the cylindrical lens required for proper vision correction.
  • Addition (ADD). The addition measurement is usually mentioned for individuals who require correction for presbyopia, an age-related condition affecting near vision. It indicates the additional power needed for reading or other close-up activities.

Additional Considerations Of A Contact Lens Prescription

A contact lens prescription is specifically tailored for contact lenses, which are directly placed on the surface of the eye to correct vision. Unlike eyeglasses, contact lenses require additional measurements and considerations due to their proximity to the eye and their direct interaction with the cornea.

Components of a Contact Lens Prescription

In addition to the elements found in an eyeglass prescription, a contact lens prescription includes specific measurements and information related to the fit and suitability of the contact lenses. These components may vary slightly depending on the type of contact lenses prescribed, such as soft lenses, rigid gas permeable (RGP) lenses, or specialty lenses. However, the core components of a contact lens prescription typically include:
  • Base Curve (BC). The base curve measurement determines the curvature of the contact lens. It ensures that the lens fits the shape of the cornea properly, promoting comfort and optimal vision correction. Base curve values usually range from 8.0 to 10.0 millimeters (mm), with smaller numbers indicating steeper curves and larger numbers indicating flatter curves.
  • Diameter (DIA). The diameter measurement represents the size of the contact lens. It ensures that the lens covers the cornea adequately while allowing for proper oxygen flow and tear exchange. Diameter values typically range from 13.0 to 15.0 mm.
  • Brand and Material. The contact lens prescription specifies the brand and material of the prescribed lenses. Different contact lens brands offer various materials, designs, and features, allowing for customization based on the individual's needs and preferences.
  • Prescription Expiration. A crucial difference between a contact lens prescription and an eyeglass prescription is the expiration date. Contact lens prescriptions are generally valid for one year from the date of the eye exam. Regular eye examinations are necessary to monitor eye health, evaluate the suitability of the lenses, and make any necessary adjustments to the prescription.

The Importance of a Contact Lens Eye Exam and Fitting

Obtaining a contact lens prescription requires a comprehensive eye examination and a fitting by an optometrist. This specialized examination evaluates the overall health of your eyes, determines your visual needs, and ensures that contact lenses are a safe and suitable option for you. During the contact lens fitting, the optometrist will take additional measurements to determine the appropriate lens parameters for your eyes. They will assess factors such as corneal shape, tear film stability, and eye health to ensure a proper fit and minimize the risk of complications or discomfort. The fitting process may involve trial lenses, where different options are tested to determine the best fit and vision correction. Following the fitting, the optometrist will provide specific instructions on how to properly insert, remove, clean, and care for the contact lenses. They will also schedule follow-up visits to monitor your eye health and make any necessary adjustments to the prescription or lens parameters.

Safety and Eye Health Considerations

Contact lenses require proper care and hygiene to maintain eye health and prevent complications. Unlike eyeglasses, contact lenses come into direct contact with the surface of the eye, which increases the risk of infections, corneal abrasions, and other issues if not handled and maintained correctly. It is essential to adhere to the prescribed wearing schedule, follow optometrist instructions for cleaning and disinfecting the lenses, and replace them as recommended. Regular eye exams are also essential, as they allow the optometrist to monitor your eye health, evaluate the effectiveness of the contact lenses, and address any concerns or changes in your vision.

There are significant differences between a contact lens prescription and an eyeglass prescription. An eyeglass prescription is specifically designed for lenses that are placed within a frame, while a contact lens prescription requires additional measurements and considerations for a proper fit, vision correction, and eye health.

Schedule an eye exam or contact lens eye exam and fitting with our Edmonton optometrists today!

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A contact lens prescription is valid for one year.

Proximity (i.e., distance) of the corrective lens to the eye is the main reason why contact lens and eyeglass prescriptions differ.