What must be added to the center thickness to predict lens thickness using the sag formula?

To predict lens thickness using the sag formula, adding a specified center thickness is essential because the sag formula calculates the depth of the lens based on its curvature, diameter, and the height of the refracted light. The center thickness serves as a critical reference point for this calculation, determining the overall lens thickness from its center to the edge.

The sag formula primarily considers the curvature of the lens and the distance from the vertex to the lowest point of the lens surface, but starting from a specific center thickness allows for a more accurate prediction of the final lens thickness. This is crucial for ensuring that the lenses fit correctly in the eyewear frame and meet the optical requirements of the wearer.

Other options do not serve as necessary components for this prediction. For instance, a calculated adjustment factor could be valuable in some contexts, but it is not a fundamental requirement for using the sag formula itself. The average lens thickness provides a general idea but does not provide the precise measurements needed for accurate calculations. Frame size, while relevant in a broader context of lens fitting, does not directly impact the mechanical sag calculations necessary for determining lens thickness in this scenario.