Can You Convert a Glasses Prescription to Contacts? - Blue Planet Optics

Switching from glasses to contact lenses can be an appealing option for those seeking convenience and a more natural look. However, understanding how to convert glasses to contact lenses involves more than just using the same numbers.

In this article, we will go into detail about glasses and contact lens prescriptions to understand why a direct conversion isn't possible and what steps you need to take to get the right contact lenses in such a case.

Are Glasses Prescriptions and Contact Lens Prescriptions the Same?

In short, no. Glasses and contact lens prescriptions are not identical. While both glasses and contact lenses serve the purpose of correcting vision, their prescriptions differ in many ways. This is primarily due to the difference in how they sit on or in front of your eyes.

Glasses typically rest about 12-14 millimeters away from your eyes, while contact lenses sit directly on the surface of your eye. This difference in distance, known as the vertex distance, affects how light is refracted into your eye and, consequently, the power needed to correct your vision.

Several key factors distinguish contact lens prescriptions from glasses prescriptions. For stronger prescriptions, contacts may need a different power than glasses due to the vertex distance. 

Contacts also require additional measurements like the base curve, which determines how the lens fits your eye's shape, and the diameter, which specifies the lens width. Moreover, prescriptions often specify a particular brand of contacts or material, as different lenses have unique properties. Finally, contact prescriptions typically expire after one year, while glasses prescriptions may remain valid for two years.

Is Astigmatism Corrected the Same in Glasses and Contacts?

Astigmatism is a common eye condition where your eye's surface isn't perfectly round, causing blurred vision.

In glasses, astigmatism is corrected using cylindrical lenses that can be precisely oriented. Contact lenses for astigmatism, called toric lenses, work differently and have some limitations. They're not available for very mild astigmatism, (less than 0.75 diopters), and the maximum correction is usually less than what glasses can provide (2.25 diopters). Also, astigmatism correction in contacts is usually in 0.50 diopter steps, while glasses can be more precise with 0.25 diopter steps.

Due to these differences, your eye care professional may need to adjust the astigmatism correction in your contact lens prescription compared to your glasses prescription.

Is the Same Information Included in Glasses and Contact Prescriptions?

While there is some overlap in the information provided in glasses and contact lens prescriptions, there are also important differences. Both include sphere (SPH) for correcting nearsightedness or farsightedness, cylinder (CYL) for astigmatism correction, axis for orientation of astigmatism correction, add power for bifocals or multifocals, and OD/OS designations for right and left eyes.

However, prescriptions for contacts go further, including details about the base curve (BC) to match your eye's curvature, diameter (DIA) for the size of the lens, and specific brand or material recommendations for proper fit and comfort.

Can You Order Contacts Without a Prescription?

In the United States, it's illegal to order contact lenses without an eye care professional's valid, up-to-date prescription. This rule applies even to non-corrective colored contacts, as all contact lenses are considered medical devices by the FDA.

Trying to order contacts without a prescription or using an outdated prescription can lead to serious eye health issues. Your prescription ensures that your contacts fit properly and provide the correct vision correction.

I Wear Glasses, Can I Switch to Contacts?

If you're considering switching from glasses to contacts, the first step is to schedule a new eye exam specifically for contacts with your eye doctor. They will assess whether your eyes are suitable for contact lenses, take additional measurements, discuss your lifestyle and preferences, and allow you to try on sample lenses to ensure comfort and proper fit.

They will also examine the health of your cornea to make sure it can safely accommodate contact lenses. Therefore, this can often be done as part of your annual comprehensive eye exam.

Most people can wear contact lenses, but they might not be suitable if they have certain eye health issues or have had specific eye surgery. Your eye doctor will advise you on your options.

How to Calculate Contact Lens Power from Glasses

An eye care professional typically uses a precise formula to adjust the sphere power, especially for higher prescriptions, to account for the difference in distance between the glasses and the cornea. This adjustment, known as vertex conversion, is necessary because glasses sit about 12-14 mm away from your eyes, whereas contact lenses rest directly on the cornea. For prescriptions with powers above ±4.00D, this significantly impacts the accuracy of the lens power.

The conversion process also involves refining the cylindrical power and axis values for astigmatism to match the parameters available in contact lenses. Unlike glasses, contact lenses have specific limitations and options for cylindrical correction, which means the cylindrical power and axis might need to be adjusted slightly. To ensure the most accurate fit and vision correction, eye care professionals often use trial lenses. These lenses allow for practical assessment and fine-tuning based on how the lenses fit your eyes and how well they correct your vision, providing a tailored solution that over-the-counter conversions cannot achieve.

Using a Contact Lens Conversion Chart

A contact lens conversion chart can be a helpful tool. These charts provide a quick reference for adjusting the sphere power from your glasses prescription to an approximate contact lens power. However, these charts are not a substitute for professional lens fitting, as they do not account for individual variations in corneal curvature and lens fit.

While it's not possible to directly convert your glasses prescription to a contact lens prescription, understanding the differences between the two can help you navigate the process of switching to contacts. Remember, the key to successful contact lens wear is a proper contact lens fitting and prescription from a qualified eye care professional.

If you're considering making the switch to wearing contacts, book an appointment with your eye doctor to discuss your options and get a contact lens exam. The right care and prescription will offer you visual acuity and freedom from glasses in your daily life.

Want to know more about contact lenses? Visit our blog for expert insights and tips!