Above about ±4.00 D, a contact lens needs a different power than the glasses, because it sits on the cornea instead of roughly 12 mm in front of it. This converter applies the vertex formula to turn a spectacle prescription into the equivalent contact lens power. Enter the sphere (and cylinder) and the vertex distance, and it returns the power to order.
The Conversion
F_cl = F_s / (1 − v × F_s), where F_s is the spectacle power in dioptres, v is the vertex distance in metres (12 mm = 0.012 m), and F_cl is the contact lens power.
Moving the correction from the spectacle plane onto the cornea shifts the power toward plus: minus powers become less minus, plus powers become more plus. As EyesOnEyecare’s vertex reference notes, the conversion only matters once the spectacle power passes about ±4.00 D; below that, the change is smaller than a single 0.25 D step.
Worked Examples
- A −8.00 D spectacle lens at 12 mm: −8.00 / (1 − 0.012 × −8.00) = −8.00 / 1.096 = −7.30 D at the cornea (less minus).
- A +8.00 D spectacle lens at 12 mm: +8.00 / (1 − 0.012 × 8.00) = +8.00 / 0.904 = +8.85 D (more plus).
The stronger the prescription, the larger the gap: at −12.00 D the contact is about −10.50 D. For an astigmatic prescription, convert each principal meridian separately (the sphere, and the sphere plus the cylinder), then rewrite the result as sphere, cylinder and axis.
Vertex Distance and the ±4.00 D Threshold
The conversion uses the spectacle vertex distance, the gap between the back of the lens and the cornea, conventionally 12 mm. Below ±4.00 D the difference between glasses and contact power stays inside a single 0.25 D step, so most fitters convert only at ±4.00 D and above. The same vertex physics, applied between two spectacle positions rather than all the way to the cornea, drives the vertex distance compensation guide and its calculator. Lens-to-eye geometry is also what Optogrid captures from a photo when measuring spectacles.
Frequently Asked Questions
How do you convert a glasses prescription to contacts?
Apply the vertex formula F_cl = F_s / (1 − v × F_s), with v the vertex distance in metres (usually 0.012). Minus powers become less minus and plus powers more plus. Below about ±4.00 D the change is smaller than a 0.25 D step and can be ignored.
At what power does vertex conversion matter?
About ±4.00 D. Below that, the difference between spectacle and contact power is under 0.25 D. By −8.00 D the contact is roughly 0.70 D less minus, and the gap grows with power.
Why is a contact lens weaker than the glasses for a myope?
Because it sits closer to the eye. Moving a minus correction onto the cornea shifts its required power toward plus, so a −8.00 D spectacle becomes about −7.30 D in a contact lens.
What vertex distance should I use?
The standard assumption is 12 mm, the typical gap from the back of a spectacle lens to the cornea. Use the measured vertex if you have it; the conversion is most sensitive at high powers.
How do I convert an astigmatic prescription?
Convert each principal meridian on its own (the sphere, and the sphere plus the cylinder), then rewrite as sphere, cylinder and axis. The axis itself does not change.
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