Which statement is true about comparing spectacle and contact lens power for the same refractive error?

Which statement is true about comparing spectacle and contact lens power for the same refractive error?

• A. Contact lenses generally require more plus power than spectacles.
• B. Spectacle lenses generally require more plus power than contact lenses.
• C. They require the same power.
• D. It varies randomly.

Answer: (A)

Vertex distance and where the lens sits dramatically change the power that actually reaches the eye. Spectacle lenses sit some distance from the cornea, so the power you prescribe isn’t delivered at the eye itself in the same way as a lens that sits directly on the cornea (a contact lens). The effective power at the eye can be calculated with the idea that eye-receptor vergence changes with distance. For a given refractive error, the power at the eye with spectacles is different from the lens power on the cornea. With hyperopia (positive power), the effective power at the eye is greater than the spectacle’s nominal power because of the vertex distance. To achieve the same correction at the retina, you need a smaller spectacle power than the contact lens power. In numbers, a hyperopic error of +3.00 D might be corrected with about +2.90 D as a spectacle to yield +3.00 D at the retina, while a contact lens would be +3.00 D. Thus, contact lenses require more plus power than spectacles for the same refractive error. This contrast is reversed for myopia, where the spectacle power would typically need to be more minus than the contact lens power to achieve the same retinal correction, illustrating how vertex distance shifts the comparison based on sign.

Vertex distance and where the lens sits dramatically change the power that actually reaches the eye. Spectacle lenses sit some distance from the cornea, so the power you prescribe isn’t delivered at the eye itself in the same way as a lens that sits directly on the cornea (a contact lens). The effective power at the eye can be calculated with the idea that eye-receptor vergence changes with distance.

For a given refractive error, the power at the eye with spectacles is different from the lens power on the cornea. With hyperopia (positive power), the effective power at the eye is greater than the spectacle’s nominal power because of the vertex distance. To achieve the same correction at the retina, you need a smaller spectacle power than the contact lens power. In numbers, a hyperopic error of +3.00 D might be corrected with about +2.90 D as a spectacle to yield +3.00 D at the retina, while a contact lens would be +3.00 D. Thus, contact lenses require more plus power than spectacles for the same refractive error.

This contrast is reversed for myopia, where the spectacle power would typically need to be more minus than the contact lens power to achieve the same retinal correction, illustrating how vertex distance shifts the comparison based on sign.