If you wear both glasses and contact lenses, you might already know that your glasses prescription doesn’t directly translate to contact lenses. Using the same prescription for both can lead to issues, as the two require different measurements for optimal vision. So how do you convert your glasses prescription to a contact lens prescription? Let’s break it down into simple steps.
Why Is Prescription Conversion Necessary?
The strength of your glasses prescription is often higher than that of contact lenses. This is due to the vertex distance—the space between your glasses and your eyes (about 12mm). Glasses sit on your nose, while contact lenses rest directly on your cornea. Because of this difference, a direct transfer of your glasses prescription to contact lenses would result in incorrect lens power.
Wearing lenses with the wrong prescription can cause blurry vision, discomfort, and even eye strain. Properly converting your prescription ensures clear and comfortable vision while wearing contacts.
How to Convert Your Glasses Prescription
You can calculate your contact lens power using a simple formula or a conversion chart.
Conversion Formula
The formula for converting glasses prescription to contact lens power is:
Contact Lens Power = Glasses Power ÷ [1 – (Vertex Distance × Glasses Power)]
Example Calculation
If your glasses prescription is -6.00D:
Contact Lens Power = -6.00 ÷ [1 – (0.012 × -6.00)]
Contact Lens Power = -5.66D
Since contact lens power is typically rounded to the nearest 0.25D, your contact lens prescription would be -5.75D.
This demonstrates why higher prescriptions often need adjustments. However, if your glasses prescription is -4.00D or lower, the difference is minimal, and you can typically use the same value for both glasses and contact lenses.
Conversion Chart for Nearsightedness
A conversion chart is a quick way to find your contact lens power without calculations. To use it:
1. Locate your glasses prescription on the chart.
2. Find the corresponding contact lens power.
Note: If your glasses prescription is -4.00D or below, no conversion is typically needed. You can use the same power for your contact lenses.
By understanding and correctly applying the conversion process, you can choose the right lens power and maintain the beauty and health of your eyes.