From the prescription, the lens material and the base curve of the blank to the lap curves for the back surface, the centre and edge thickness and what your lens clock should read.
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How the curves are calculated
A semi-finished blank has its front curve, the base curve, already made. Surfacing grinds the back so that the lens gets the prescribed power. The calculator works back from the power you need:
- Front: the base curve as marked on the blank (1.53 scale) becomes the true surface power of the material: base curve × (n − 1) / 0.53.
- Thickness: the front curve is moved to the back vertex with the thick-lens formula F1 / (1 − t/n · F1). For plus lenses the centre thickness follows from the edge thickness and the sags of both surfaces, calculated again until it no longer changes.
- Back: back curve = prescribed power − effective front power, per principal meridian. A toric prescription is set in minus-cylinder form, so the toric curve is on the back.
- Lap: laps and lens clocks are marked in a tool index, usually 1.530. The lap curve is the back curve × (tool index − 1) / (n − 1), rounded to the steps of your lap set.
Why a lens clock reads a different number
A lens clock (Geneva lens measure) measures the sag of a surface and shows it as a power for index 1.53. On CR-39 (1.498), polycarbonate (1.586) or high-index material the reading is the curve, not the true surface power. The calculator gives the readings you should see on the finished lens, so you can check the front and both back meridians.
Choosing the base curve
Vogel’s rule is a common starting point: for plus lenses the spherical equivalent + 6.00 D, for minus lenses half the spherical equivalent + 6.00 D. The calculator suggests it and the nearest common blank base; the blank series of your lens supplier decides.
For the optical workshop
Check every finished lens with a lensmeter. For edging, grooving and fitting see our optical workshop equipment, or ask us.
Frequently asked questions
What is the tool index?
The index in which laps, lens clocks and sag gauges are marked, usually 1.530 (some older glass laps: 1.523). It lets one lap set serve every material: the calculator converts the true back curve of the material to the lap curve.
Where does the cylinder go?
In minus-cylinder form on the back. The base curve of the toric lap lies along the cylinder axis; the cross curve, the steeper one, lies at 90° to it.
Why is my plus lens thicker than expected?
A plus lens is thinnest at the edge. The centre thickness follows from the diameter: a smaller uncut size or edged lens gives a thinner centre.
Are prism and freeform designs included?
No. The calculator is for spherical and toric surfaces with a round, uncut lens. Prism, decentration and aspheric or freeform designs need the data of your generator or lens supplier.