The vertex distance calculator gives the power at the cornea of a spectacle lens, as needed for a contact lens, and the power needed at a new vertex distance.
How the calculation works
- Enter the lens power and the vertex distance.
- Enter a new distance if needed.
- Read the power at the cornea and the power at the new distance.
Example
A lens of −8.00 D at 12 mm is equivalent to −8 / (1 − 0.012 × (−8)) = -7.30 D at the cornea, a change of 0.70 D. If the glasses sit at 14 mm instead, the lens needed is -8.13 D.
Why it matters
The calculator helps opticians, optometrists, ophthalmologists and students who convert a glasses prescription to contact lenses or who adjust a frame that sits closer or farther from the eye.
Good to know
The formula uses the thin lens approximation and the distance from the back surface of the lens to the front of the cornea, normally measured with a distometer. Contact lens fitting also depends on the base curve, the diameter and the tear layer, which makes a difference of 0.25 to 0.5 diopter in the final lens. For high plus lenses such as after cataract surgery, the correction is large, and for high minus lenses the contact lens is weaker, which means that a patient moving to contacts has more accommodative demand at near. In astigmatic prescriptions, correct each principal meridian separately and recompute the cylinder. The result is rounded to the available steps of 0.25 diopter, or 0.50 above 10 diopters. Always give the vertex distance used when a power is quoted, because a lens power without it can be misread.
Related tools
The spherical equivalent calculator converts prescription forms, the keratometry calculator treats corneal power, the reading glasses calculator treats near vision and the visual acuity calculator converts acuity.