Contact Lens Power
Optics and Optometry
Spectacle prescription to contact lens power and back, at any vertex distance.
How it is calculated
- This is vertex compensation with the lens moved all the way to the cornea, so d is the vertex distance itself — 12 mm by convention unless it was measured.
- Contact power = spectacle power ÷ (1 − d × spectacle power).
- Each principal meridian is converted separately, so a toric prescription's cylinder changes as well as its sphere.
- The difference becomes clinically real from about ±4.00 D and is substantial past ±8.00 D — a high myope routinely needs a full dioptre less minus in contact lenses.
- The answer is given both exactly and rounded onto the 0.25 D steps a lens is actually made in. Many soft lenses step in 0.50 D above ±6.00 D, so the available parameter may be coarser still.
References
- Standard contact lens practice — the back vertex power relationship applied at the corneal plane.
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.