Back vertex power
Spectacle Rx to the power needed at the cornea (vertex distance compensation).
mm
About this calculator
A spectacle lens sits about 12 mm in front of the cornea, so a contact lens needs a different power to put the far point of the eye in the same place. Plus prescriptions need more plus at the cornea, and minus prescriptions need less minus.
The difference is small below ±4.00 D and is usually ignored there. Above it, each principal meridian is converted on its own, and the two results give the sphere and cylinder at the cornea.
Formula
Fc = F / (1 − d·F), with d the vertex distance in meters
Worked example
For −6.00 −1.50 × 180 at 12 mm, the 180 meridian gives −6.00 / 1.072 = −5.60 D and the 090 meridian gives −7.50 / 1.090 = −6.88 D, so the power at the cornea is −5.60 −1.28 × 180.
Sources
- Contact Lenses in Ophthalmic Practice, ch. 7, question 9 (vertex distance)
- System for Ophthalmic Dispensing, ch. 14, Effective Power