What is the term for the distance from the anterior surface of the eye to the back surface of the spectacle?

The term that describes the distance from the anterior surface of the eye to the back surface of the spectacle is known as Vertex Distance. This measurement is crucial in the fitting of spectacles because it directly affects the effective power of the lenses when worn. If the vertex distance is too long or too short, it can result in optical errors and discomfort for the wearer, as the lenses may not perform as intended.

Pupillary Distance measures the distance between the pupils and is essential for aligning the optical center of the lenses with the lines of sight. Focal Length refers to the distance from a lens to the point where parallel rays of light converge and is not directly related to the positioning of spectacles on the face. Optical Center Distance is similarly related to how lenses are positioned but does not define the distance between the eye and the back of the spectacle. Thus, Vertex Distance is specifically defined and critical for ensuring optimal visual acuity with eyeglasses.