Definitions of Optical Terminology

PD – Pupillary Distance or interpupillary distance (IPD) is the distance measured in millimeters between the centers of the pupils in each eye.  The PD measurement is used primarily in fabricating custom prescription eyeglasses.

Pantoscopic Tilt - the angle between the plane of the lens and eyewear frame front and the frontal plane of the face when the upper edge of the lens is slightly farther away from the frontal plane than the lower edge of the lens.  It enlarges the field of view for reading but creates a slight amount of astigmatism.

Vertex Distance -  the distance between the back surface of a corrective optical lens (i.e. glasses or contact lenses) and the front of the cornea. Increasing or decreasing the vertex distance changes the effective focal power of the lens system.