Knapp's law is named after a law that relates retinal image magnification to placement of a lens at what specific location?

• A. At the secondary focal point
• B. At the primary focal point
• C. At the corneal apex
• D. At the pupil plane

Answer: (B)

Knapp's law describes how the size of the retinal image produced by a spectacle lens depends on where that lens sits relative to the eye. The key idea is that the retinal magnification is simplest and most predictable when the lens is placed at the eye's front (primary) focal point. At this position, parallel light rays entering the eye from distant objects are refracted in such a way that the eye sees a magnified (or minified) image in a consistent, angle-based manner that follows the lens power. If the lens were located at other positions—such as the back focal point, the corneal apex, or the pupil plane—the geometry becomes more complex and the simple relationship described by Knapp's law no longer applies. So the placement at the primary focal point is the situation Knapp's law uses to relate retinal image magnification to lens position.