The primary focal point distance described in Knapp's law is approximately which value in millimeters?

• A. 10
• B. 16.67
• C. 20
• D. 25

Answer: (B)

Knapp's law uses a practical reference distance from the eye to the corrective lens to describe how vertex distance affects magnification. For a typical eye, this primary focal point distance is about two-thirds of the eye’s length, which works out to roughly 16.7 millimeters. This value is a convenient, widely used rule of thumb for estimating how changing the gap between eye and lens changes the image size. While actual eyes vary, 16.7 mm is the standard figure that best fits the common teaching and calculations, making it the correct choice. The other options don’t match this commonly cited reference distance.