Convert a spectacle prescription to contact lens power per meridian, switch between plus and minus cylinder and see both optical crosses.
Convert a prescriptionNothing you enter is sent or stored.
How the vertex conversion works
A lens acts differently at the cornea than 12 mm in front of it. The effective power at the new position is F′ = F / (1 − d·F), with d the distance moved towards the eye in metres. Minus lenses need less power at the cornea, plus lenses more.
For a toric prescription the conversion is done on each principal meridian separately: the axis meridian carries the sphere, the meridian at 90° the sphere plus the cylinder. Converting only the sphere gives the wrong cylinder at higher powers. The calculator shows the exact power per meridian and the power rounded to 0.25 D steps.
Transposition: plus and minus cylinder
The same lens can be written in two ways. To transpose: add the cylinder to the sphere, change the sign of the cylinder and turn the axis by 90°. Example: −2.00 / −1.50 × 180 is the same lens as −3.50 / +1.50 × 90.
Keratometry: radius and power
Keratometers and autorefractor-keratometers report the corneal curvature as a radius in millimetres, a power in dioptres, or both. With the keratometric index 1.3375 the power is 337.5 divided by the radius: 7.80 mm is 43.27 D.
Instruments for refraction and lens checks
- Lensmeters read sphere, cylinder, axis and prism of the glasses a patient brings in.
- Phoropters and trial lens sets for the subjective refraction.
- Autorefractors with keratometer for objective refraction and corneal curvature in one measurement.
We select the instruments for your room and budget and quote within 24 hours. Ask us.
Frequently asked questions
From what power does the vertex distance matter?
From about ±4.00 D the change comes close to 0.25 D and starts to affect the lens you order; below that it stays under one step.
Which vertex distance should I use?
The distance at which the refraction was measured: the back of the trial lens or phoropter lens to the cornea. 12 mm is the usual value; the calculator lets you set 0 to 25 mm.
Why does the contact lens cylinder differ from the spectacle cylinder?
Because the two principal meridians have different powers, the vertex distance changes them by different amounts. For minus prescriptions the cylinder at the cornea is usually a little lower.
Is anything I enter stored?
No. The calculator runs in your browser; nothing you enter is sent to us or saved.