Why Does My Contact Lens Prescription Look Different from My Glasses?

When you receive a prescription for contact lenses and glasses, you may notice that the numbers don’t match. This often leads to confusion and concern—are you being prescribed the correct power? Will you see as clearly in your contacts as you do in your glasses?

At Harmony Vision Optometry and Vision Therapy Clinic, we get this question frequently. The answer lies in the fundamental differences between how glasses and contact lenses work. While both aim to correct your vision, the way they interact with your eyes leads to differences in prescription values.

In this blog, we’ll break down why your prescriptions differ and help you understand how your optometrist ensures you get the best vision possible—whether you wear glasses, contact lenses, or both.

Different Numbers, Same Outcome

To illustrate this, let’s look at an example:

Imagine you have a high minus prescription of -6.50 diopters in glasses. When you switch to contact lenses, your prescription might change to -6.00 diopters. This discrepancy might seem alarming, but rest assured—it’s completely normal. The numbers may be different, but the effective correction remains the same.

Understanding Vertex Distance

One of the main reasons for this difference is vertex distance—the space between the back of your glasses lens and the front of your eye.

• Glasses sit in front of your eyes (typically 8-14mm away), which means light bends slightly differently before reaching your eye.
• Contact lenses sit directly on your eye, eliminating this distance.

For higher prescriptions (usually above ±3.50 diopters), this vertex distance significantly affects the effective power of the lens. Optometrists make adjustments to account for this, ensuring that the prescription you receive in contacts provides the same level of clarity as your glasses.

Other Differences in Contact Lens Prescriptions

Beyond the numbers, contact lens prescriptions contain other critical specifications that don’t appear in glasses prescriptions:

• Lens Diameter & Base Curve: These measurements ensure the lens fits properly on your cornea.
• Lens Material & Brand: Different brands have different oxygen permeability, comfort levels, and moisture retention.
• Astigmatism Adjustments: If you have astigmatism, toric contact lenses have specific axis measurements that may slightly differ from your glasses.

Presbyopia: Glasses vs. Contact Lenses

If you’re over 40, you may be dealing with presbyopia, where near vision starts to decline.

• Glasses often include a "near addition" power, increasing in strength as you look down through progressive lenses.
• Multifocal contact lenses work differently—they incorporate both distance and near prescriptions within the same lens, requiring your brain to adapt to selectively focusing at different distances.

Because of this, contact lens prescriptions may not specify a precise "add power" like glasses do. Instead, they are categorized as Low, Medium, or High Add depending on your vision needs.

Astigmatism & Toric Contact Lenses

Astigmatism occurs when the eye is shaped more like a rugby ball rather than a soccer ball, leading to different curvatures that need correction.

• In glasses, astigmatism is corrected using cylinder power and axis—precise numbers down to a single degree.
• In contact lenses, manufacturers offer fewer options (typically in 10-degree steps) and limited cylinder powers. If your exact prescription isn’t available, optometrists adjust your lens power to provide the best possible clarity.
• Additionally, toric contact lenses can rotate on the eye, requiring adjustments to ensure stability and proper vision correction.

These small changes might make your contact lens prescription look different, but they are carefully calculated to give you the best vision possible.

Trust Your Optometrist

At the end of the day, the differences between your glasses and contact lens prescriptions are just optical adjustments to ensure that your vision is as sharp and comfortable as possible.

✔️ The numbers may look different, but the visual outcome remains the same.
✔️ Contact lenses require additional fitting considerations that glasses don’t.
✔️ Adjustments for vertex distance, astigmatism, and presbyopia explain why your prescriptions aren’t identical.

If you have any further questions or need personalized advice about your prescription, feel free to contact us at Harmony Vision Optometry. Our team is happy to help you achieve clear, comfortable vision—whether you wear glasses, contacts, or both!