Why Your Eyeglasses Prescription Won't Work for Contact Lenses

About to order contact lenses using your glasses prescription? This is a critical mistake that could cost you money and compromise your eye health.

You're likely seeing contact lens fields asking for "Base Curve" and "Diameter," and your astigmatism numbers don't seem to match up. There's an important reason for this.

You cannot use your glasses prescription to purchase contact lenses. Here's why this matters for your eye health.

This is the most common error we see Australians make when ordering contacts online. Using the wrong prescription leads to uncomfortable lenses, compromised vision, and potential eye health complications.

Why Glasses Prescriptions Are Different from Contact Prescriptions

1. Vertex Distance

Your glasses sit approximately 12mm from your eyes. Contact lenses sit directly on the eye surface. This distance fundamentally changes the optical power required for clear vision.

This gap (vertex distance) requires precise power calculations, especially for prescriptions stronger than ±4.00 diopters. Only a qualified optometrist can determine the correct power conversion—it cannot be estimated.

2. Contact Lenses Are Medical Devices Requiring Precise Fitting

Your glasses prescription addresses only vision correction. It provides no information about your eye's physical dimensions or health requirements.

Contact lenses must be precisely fitted to your corneal shape and size. Incorrect fitting can result in:

• Discomfort and irritation
• Reduced oxygen permeability
• Compromised vision quality
• Risk of corneal complications and infection

Eyeglasses vs. Contact Lens Prescription

Glasses Prescription Includes:

• Sphere (SPH): Primary vision correction power
• Cylinder (CYL): Astigmatism correction power
• Axis: Angle of astigmatism correction

Contact Lens Prescription Additionally Requires:

• Base Curve (BC): Precise curvature measurement to match your cornea (e.g., 8.6mm)
• Diameter (DIA): Lens width for proper corneal coverage (e.g., 14.2mm)
• Brand & material: Specific material chosen for your eye's oxygen and moisture requirements

The Astigmatism Problem: Why Direct Conversion Isn't Possible

Australians with astigmatism often encounter these specific issues:

Limited cylinder powers available - Your glasses may have a -0.50 cylinder correction. Contact lenses are mass-produced and may not be available in this exact power. An optometrist must professionally determine the appropriate alternative.

Standardised axis parameters - Glasses can be manufactured to precise angles like 177° or 94°. Contact lenses are produced in standardised increments (typically every 10°: 90°, 100°, 110°). Only a qualified practitioner can determine which parameter provides optimal vision stability.

The Professional Process: Getting Your Contact Lens Prescription

Step 1: Contact Lens Consultation

A comprehensive appointment where your optometrist will:

• Measure your corneal curvature (Base Curve)
• Determine appropriate lens diameter
• Assess your eye health and tear film quality
• Select the optimal lens material for your needs

Step 2: Trial Fitting Period

You'll receive trial lenses to evaluate over several days of normal wear. This ensures:

• Consistent comfort throughout the day
• Stable, clear vision
• Proper lens movement and positioning

Step 3: Final Prescription

Following successful trials and a follow-up assessment, you'll receive your official contact lens prescription. You are legally entitled to a copy of this prescription, which is required for purchasing lenses.

Need Prescription Clarification? We Can Help

If you're unsure about your prescription type or next steps, our qualified team can provide guidance. Send us a clear photo of your prescription at sales@lensworld.com.au for a complimentary assessment. We'll determine whether it's for glasses or contacts and advise on the appropriate course of action.

Resources:

• https://www.webmd.com/eye-health/contacts-or-glasses
• https://www.aao.org/eye-health/glasses-contacts/how-to-read-eyeglasses-prescription
• https://www.optometry.org.au/wp-content/uploads/Publications/Optometry_Connection/Optometry-Connection-September-2021-web-compressed.pdf