In the power factor formula MP = 1/(1 - hDv), what does h represent?

• A. Vertex distance
• B. Lens thickness
• C. Back vertex distance
• D. Front vertex distance

Answer: (A)

The quantity h represents the vertex distance—the separation between the back surface of the spectacle lens and the eye (the corneal plane). This distance matters because the power of a lens is defined at the lens’s vertex, but the eye sits at some distance away, so the effective power the eye experiences changes with how far the lens is from the eye. In the expression MP = 1/(1 - hDv), the product hDv captures how shifting the lens away from or toward the eye alters the magnification/power felt at the eye. As the vertex distance changes, the adjustment term 1 - hDv changes, modifying MP accordingly. The other surface-distance terms describe different spacings, but this formula specifically uses the distance from the back surface of the lens to the eye.