In the manual lensometry equation x = f^2 Fv, what does Fv represent?

• A. Back vertex power
• B. Forward vertex power
• C. Focal length of the standard lens
• D. Back vertex distance

Answer: (A)

Back vertex power is the optical power of the lens at its back surface. In the manual lensometer equation x = f^2 Fv, the measured quantity x depends on the square of the standard lens’s focal length (f) and the back vertex power (Fv) of the test lens. This is because the measurement setup uses the back surface as the reference point, so the lens’s contribution to the formed image is defined by its power at that back vertex. The front-vertex power would refer to the front surface and isn’t what this formula uses. The focal length f belongs to the standard lens, not to Fv. And back vertex distance is a spacing, not a power, so it doesn’t fit the equation.