Will the back vertex power measurement differ from the powers measured with the lenses in the trial frame?

Will the back vertex power measurement differ from the powers measured with the lenses in the trial frame?

• A. no
• B. yes
• C. not enough information
• D. depends on frame size

Answer: (B)

Vertex distance matters: the power of a lens is specified at its plane, but what the eye actually experiences depends on how far the lens is from the eye. When you place lenses in a trial frame, they sit at a certain distance from the eye (the vertex distance). The back vertex power is the effective power at the back surface of the eye, and placing the same lens at a different distance changes its effective power due to the lens formula. The relationship P' = P / (1 − dP) (where d is the distance from eye to lens in meters) shows that as the lens is moved away from the eye, the effective power can increase or decrease depending on the sign of P. This effect is more noticeable with higher prescriptions, so the power measured in a trial frame won't equal the back vertex power unless you apply vertex distance corrections. In practice, clinicians account for this difference to ensure the final glasses deliver the intended correction.

Vertex distance matters: the power of a lens is specified at its plane, but what the eye actually experiences depends on how far the lens is from the eye. When you place lenses in a trial frame, they sit at a certain distance from the eye (the vertex distance). The back vertex power is the effective power at the back surface of the eye, and placing the same lens at a different distance changes its effective power due to the lens formula. The relationship P' = P / (1 − dP) (where d is the distance from eye to lens in meters) shows that as the lens is moved away from the eye, the effective power can increase or decrease depending on the sign of P. This effect is more noticeable with higher prescriptions, so the power measured in a trial frame won't equal the back vertex power unless you apply vertex distance corrections. In practice, clinicians account for this difference to ensure the final glasses deliver the intended correction.