Which change increases sag: increasing the overall optical diameter?

Which change increases sag: increasing the overall optical diameter?

• A. Increasing the base curve radius
• B. Decreasing OAD
• C. Keeping OAD constant
• D. Increasing OAD

Answer: (D)

Sag refers to how deep the posterior surface of a GP lens sits as you move from the center toward the edge. With a fixed curvature (the base curve), expanding the lens’s diameter means you’re bending the same curvature over a wider span. Geometrically, the deeper you go to cover a larger arc, the greater the sag becomes. A simple way to see it is that for a circle of fixed radius, increasing the diameter increases the sagita, so the edge of the lens sits more deeply relative to the center. Therefore, increasing the overall optical diameter increases sag. If the diameter is reduced, sag decreases; keeping the diameter the same keeps sag the same; changing the base curve radius to a larger value flattens the surface and reduces sag.

Sag refers to how deep the posterior surface of a GP lens sits as you move from the center toward the edge. With a fixed curvature (the base curve), expanding the lens’s diameter means you’re bending the same curvature over a wider span. Geometrically, the deeper you go to cover a larger arc, the greater the sag becomes. A simple way to see it is that for a circle of fixed radius, increasing the diameter increases the sagita, so the edge of the lens sits more deeply relative to the center. Therefore, increasing the overall optical diameter increases sag.

If the diameter is reduced, sag decreases; keeping the diameter the same keeps sag the same; changing the base curve radius to a larger value flattens the surface and reduces sag.