Reference
Common vertex distances
Vertex distance is measured from the back surface of the lens to the cornea. These are the commonly cited teaching values; an actual fit is measured, not assumed.
| Situation | Typical vertex | Notes |
|---|---|---|
| Refraction (phoropter) | about 12 to 14 mm | The distance the prescription was measured at; high-power orders should state it. |
| Typical spectacle fit | about 12 to 14 mm | Varies with frame style, bridge fit, pantoscopic tilt, and facial anatomy. |
| Close-fitting frames and sports eyewear | about 8 to 12 mm | A shorter vertex than the refraction strengthens plus, weakens minus. |
| Contact lens | 0 mm | At the corneal plane; every conversion from high-power spectacles compensates to here. |
Rule of thumb: compensation starts to matter around 4.00 D, where common fitting differences shift effective power by an eighth diopter or more. The vertex calculator shows the exact effect for any power, and the vertex guide derives the formula.
Frequently asked questions
What is a typical spectacle vertex distance?
Commonly cited fitting values run about 12 to 14 mm, varying with the frame, the bridge fit, and facial anatomy. Refraction is performed at a measured vertex, which is why high-power orders state it.
At what power should I compensate for a vertex change?
A common rule of thumb is about 4.00 D and above, where typical fitting changes move the effective power enough to matter clinically. The vertex calculator shows the exact effect for any power.