What measurement is equal to twice the longest radius from the geometric center of the lens to the apex of the edge?

The correct choice refers to the concept of effective diameter (ED), which is defined as the measurement that accounts for both the diameter of the lens as well as the curvature of the lens edges, giving a more comprehensive understanding of the lens's dimensions. Specifically, effective diameter is often used when evaluating the optical properties of lenses, and it is derived from considering the radius of curvature and the position of the geometric center.

In this context, saying that effective diameter is equal to twice the longest radius from the geometric center to the apex of the lens's edge underscores the relationship between the lens's overall profile and its effective optical functioning. This measurement is critical in customizing lenses for different prescriptions and ensuring accurate visual corrections, as it can influence how light is refracted through the lens.

The other options, while related to lens measurements, do not define a measurement equal to twice the longest radius in the same way. The diameter refers simply to the straight line across the lens that passes through its center; edge thickness concerns the thickness of the lens at its edges but does not relate directly to the longest radius; and the base curve describes the curvature of the lens surface without quantifying it in relation to radius from the center. Therefore, effective diameter correctly aligns with the described