Vertex distance is the distance between the back surface of a corrective lens, i.e. glasses (spectacles) or contact lenses, and the front of the cornea. Increasing or decreasing the vertex distance changes the optical properties of the system, by moving the focal point forward or backward, effectively changing the power of the lens relative to the eye. Since most refractions are performed at a vertex distance of 14 mm, the power of a corrective device fitted at a different vertex distance may need to be compensated to effect the same correction of the initial refraction (note: refraction is portion of an eye exam that is performed with a phoropter).
Vertex distance is important when converting between contact lens and glasses prescriptions and becomes significant if the glasses prescription is +/-4.00D or more. The focal point of the correcting lens needs to be at the far point of the eye. The formula for vertex correction is Fc=F/(1-xF), where Fc is the power corrected for vertex distance, F is the original lens power, and x is the change in vertex distance in meters.