Beta Ray Diagram Widget
How to read it
A real lens is two curved surfaces with glass between them. Light refracts at the front surface, travels through the thickness, and refracts again at the back, so a lens’s power is not just the sum of its surface powers. The number your lensometer reads is the back vertex power Fv — the vergence leaving the back surface for parallel light — and it is what goes on the Rx. By default you enter the front and back curves as true surface powers in diopters; switch “Curves entered as” to read them as lens-clock values instead. A Geneva lens clock reads the marked power, calibrated to 1.53 (or pick 1.523 / 1.70 for your clock), which drifts from the true power as the material index changes — the readout shows both. Grow the thickness or the index and watch Fv pull away from the simple surface sum. Tick “Show principal planes” to reveal H and H′, the planes the equivalent power is measured from.
Controls
Ray diagram
Readout
| Front curve | +10.00 D true · +10.60 D marked (1.53 clock) · r +50.0 mm |
|---|---|
| Back curve | -4.00 D true · -4.24 D marked (1.53 clock) · r -125.0 mm |
| Center thickness | 10.00 mm (10.0 mm) |
| Refractive index | 1.50 |
| Back vertex power Fᵥ | +6.71 D — the lensometer reading |
| Back focal distance (vertex → F′) | 148.94 mm (148.9 mm) |