Vertex distance

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).