Spectacles at 12 mm to contact lenses
The spectacle power on the left was measured with the lens 12 mm from the cornea. The two columns on the right are what the calculator gives at 0 mm.
| Spectacle power | At the cornea | Nearest 0.25 D |
|---|---|---|
| −4.00 | -3.82 D | -3.75 D |
| −6.00 | -5.60 D | -5.50 D |
| −8.00 | -7.30 D | -7.25 D |
| −10.00 | -8.93 D | -9.00 D |
| −12.00 | -10.49 D | -10.50 D |
| +4.00 | +4.20 D | +4.25 D |
| +6.00 | +6.47 D | +6.50 D |
| +8.00 | +8.85 D | +8.75 D |
| +10.00 | +11.36 D | +11.25 D |
| +12.00 | +14.02 D | +14.00 D |
Minus prescriptions need less minus at the cornea; plus prescriptions need more plus. At ±4.00 D the change is about 0.2 D, enough to move the rounded power by one step. Below that it is usually ignored.
The formula
Fnew = F ÷ (1 − d × F)
- F
- Lens power at the original position, in dioptres.
- d
- Original vertex distance minus the new one, in metres. Positive when the lens moves towards the eye: 12 mm to 0 mm is d = 0.012.
- Fnew
- Power needed at the new position. The calculator also rounds it to the nearest 0.25 D.
Worked examples
- High myope to contact lenses. −8.00 D at 12 mm: −8.00 ÷ (1 + 0.012 × 8.00) = −8.00 ÷ 1.096. The calculator shows -7.30 D, rounded -7.25 D, with the note “Vertex shift of 12mm alters power by 0.70 D”.
- A frame that sits further out. The same −8.00 D was refracted at 12 mm, but the new frame sits at 14 mm, so d = −0.002. The calculator shows -8.13 D, rounded -8.25 D.
- High plus. +10.00 D at 12 mm needs +11.36 D at the cornea, rounded +11.25 D.
Astigmatic prescriptions: one meridian at a time
The calculator takes a single power. For a sphero-cylinder, convert each principal meridian and rebuild the cylinder from the difference. Worked by hand for −6.00 / −2.00 × 180 at 12 mm: the meridians are −6.00 along 180 and −8.00 along 90. The calculator gives −6.00 → -5.60 D and −8.00 → -7.30 D, so the corneal-plane prescription is −5.60 / −1.70 × 180. Then choose the nearest parameters the toric lens is made in.
Limits
The formula treats the lens as thin, and the result is only as good as the measured vertex distance: use a distometer, or read it from the refraction head, when the power is high. A rigid lens also forms a tear lens, which this calculation does not include. See the medical disclaimer.
Watch: the same calculation in the Asaan Optics desktop app
This video converts a spectacle prescription to contact lens power in the desktop app, not in the calculator on this page. Both use the same vertex distance formula; the app applies it to each meridian of the prescription, while the calculator on this page takes one power at a time.
Video, 2 min 23 s
Working out contact lens power from glasses prescription figures means correcting each meridian for vertex distance, and this video does it once in full: -6.00 / -1.50 x 180 at 12 mm becomes -5.50 / -1.25 x 180, with a toric lens advised. Three more worked examples follow: keratometry readings between millimetres and dioptres, 6/9 across the acuity notations, and a selling price built from cost, markup, discount and tax.
Chapters
- 00:00Contact lens power
- 00:49Keratometry readings
- 01:18Visual acuity notations
- 01:36Pricing and margin
What the video covers
- Contact lens power at 12 mm: -6.00 / -1.50 x 180 becomes -5.50 / -1.25 x 180, each meridian corrected and rounded to 0.25 D
- The spherical option (-6.25 D) and why the app advises a toric lens here
- Keratometry: 337.5 divided by the radius turns 7.80 mm and 7.50 mm into 43.27 D and 45.00 D, with 1.73 D of corneal astigmatism
- Visual acuity: 6/9 is 20/30, decimal 0.67 and logMAR 0.18
- Pricing: cost 60, markup 150 percent, 10 percent discount and 20 percent tax added on top, so the customer pays 162 and the profit is 75
- The largest discount (42.9 percent) that still keeps a 30 percent margin in that example
Transcript
Four more calculators, each with one worked example. These calculators support your own judgement. They do not replace it.
Contact Lens Power converts a spectacle prescription to the power at the eye. The vertex distance is twelve millimetres. For the right eye, enter minus six, with minus one point five at one hundred and eighty. Each meridian is corrected for vertex distance and rounded to a quarter dioptre. The contact lens is weaker: minus five point five, with minus one point two five. The spherical equivalent is minus six point two five, but here the app advises a toric lens. This is a starting point. A contact lens fitting still needs a proper assessment.
Keratometry converts corneal radius to dioptres, and back. The flat reading is seven point eight millimetres. Enter a steep reading of seven point five. Three hundred and thirty-seven point five, divided by the radius, gives forty-three point two seven and forty-five dioptres. The astigmatism is one point seven three. Switch to dioptres, and the readings convert back to millimetres.
Visual Acuity converts between Snellen, decimal and log mar. For six over nine, enter only the nine. Six over nine is twenty over thirty, decimal nought point six seven, and log mar nought point one eight.
Last, Pricing and Margin. Enter a cost of sixty pounds, a markup of one hundred and fifty percent, and tax at twenty percent. The discount stays at ten. The selling price is one hundred and fifty pounds. After the discount, with tax added on top, the customer pays one hundred and sixty-two. Profit is seventy-five pounds. You could discount up to forty-two point nine percent and still keep a thirty percent margin. Four results, each updated as you type. Copy result puts the answer on the clipboard. The decision stays with you.
Questions
What vertex distance should I use if it was not recorded?
12 mm is the usual assumption for spectacles and phoropters. Above about ±5.00 D, measure it: at −10.00 D, 2 mm of difference changes the power by around 0.2 D.
Why does a myope need less minus in a contact lens?
A minus lens is effectively stronger the closer it sits to the eye, so less minus power gives the same correction at the cornea. A plus lens is effectively weaker closer to the eye, so it needs more plus.