If the index of refraction n increases while central thickness t and front surface power D1 remain constant, how does shape factor Ms change?

• A. Ms increases
• B. Ms decreases
• C. Ms remains the same
• D. Ms becomes negative

Answer: (B)

Shape factor Ms reflects how the lens’s optical power is distributed between its surfaces and where the principal planes lie. It is influenced by the index of refraction because changing n alters how strongly each surface bends light for the same geometry.

Here, central thickness and front-surface power are held fixed. To keep the front-surface power constant as n increases, the front surface curvature must become less steep (the front radius R1 must increase). A flatter front surface contributes less bending, which shifts the balance of power between the surfaces and moves the principal planes in a way that reduces Ms. In short, increasing n with t and D1 fixed makes the shape factor smaller.