In thick-lens calculations, what does Dv denote?

• A. Back vertex power
• B. Front vertex power
• C. Total power
• D. Object distance power

Answer: (A)

Dv denotes the back vertex power—the dioptric strength of a thick lens measured at the back surface (back vertex). In thick lenses, the power you get depends on which surface or plane you measure it from, since the lens has thickness and light refracts at both surfaces. The back-vertex power is what matters when predicting how the lens will alter vergence as rays exit the lens and head toward the eye, and it’s the quantity used when converting prescriptions to the eye’s working distance. The front-vertex power would be the analogous measure at the front surface, while the total power is the overall dioptric effect of the whole system, not tied to a single vertex. Object distance power isn’t a standard term.