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Center thickness is the measurement of a prescription lens at its thinnest point, which is usually the geometric center. This measurement is important because it directly impacts the final weight, strength, and safety of the finished spectacle lens.
read more about center thickness ...Center thickness varies significantly by prescription type. For a farsighted person (plus prescription), the center of the lens is the thickest point. For a nearsighted person (minus prescription), the center of the lens is the thinnest point.
Even at its thinnest point, a lens must meet a minimum thickness requirement set by safety standards. This requirement makes sure the lens is strong enough to resist impact and prevent cracking when it is mounted into the frame.
The optician controls center thickness by selecting the index of refraction and the base curve. Higher index materials allow for thinner centers. Also, adjusting the lens base curve can sometimes reduce the center thickness slightly while maintaining optical quality.
The thickness of your backup glasses affects their comfort. A thicker lens is heavier. Controlling center thickness helps make sure your backup frames are lightweight and comfortable, providing maximum relief when you temporarily switch from contacts to glasses.
Yes, contact lenses are extremely thin in the center to maintain comfort and oxygen flow, but the thickness is measured in fractions of a millimeter.
Yes, selecting a high-index material is the best way to reduce center thickness for plus prescriptions and edge thickness for minus prescriptions.
Labs use a specialized electronic caliper (called a gauge) to measure the thickness of the lens blank before and after surfacing.
For farsighted (+) patients, "Center Thickness" (CT) is the primary concern for weight. If your glasses feel like they are falling off your face, ask about "digital surfacing" or "pre-cal," which can reduce CT by up to 20% compared to standard surfacing.
AAO. Optics of the Eye and Lenses (aao.org). 2024.
Review of Optometry. Managing High Plus Prescriptions (reviewofoptometry.com). 2024.
The Vision Council. Lens Surfacing and Pre-Calc (thevisioncouncil.org). 2023.
ABDO. Calculating Lens Thickness (abdo.org.uk). 2024.