Vertex Distance Compensation
Optics and Optometry
What a prescription becomes when the lens moves closer to or further from the eye.
How it is calculated
- New power = old power ÷ (1 − d × old power), where d is the distance the lens moves toward the eye, in metres.
- A myope needs less minus as the lens comes closer, and a hyperope needs more plus. A −10.00 D spectacle at 12 mm becomes −8.93 D at the cornea; a +10.00 D becomes +11.36 D.
- A sphero-cylinder has to be compensated one principal meridian at a time, because the two meridians carry different powers and shift by different amounts. Compensating only the sphere leaves the cylinder wrong.
- Moving a lens along its own optical axis cannot rotate its meridians, so the axis never changes.
- Below about ±4.00 D the shift is smaller than a single 0.25 D step, which is why compensation is conventionally reserved for stronger prescriptions.
References
- Standard ophthalmic optics — effective power / back vertex power relationship.
About these tools
OcuLearning Clinical Tools is part of an educational platform. These calculators are provided for learning and reference, and are not medical advice, a diagnosis, or a treatment recommendation.