Which measurement is crucial for achieving the optimal fit of the lens into the frame?

The effective diameter (ED) is a vital measurement for ensuring that the lens fits properly within the frame. It is defined as the diameter of the smallest circle that can completely fit within the lens shape. This measurement is crucial because it helps opticians determine if the lens will fit without being cut too close to the edge of the frame, which could compromise both aesthetics and structural integrity. An accurate ED ensures that the lenses are not only securely held in place but also allows for the necessary thickness and edge treatment that vary depending on lens design and prescription requirements.

In contrast, the other measurements, while important in their own right—such as the A and B measurements, which relate to the width and height of the frame—do not provide the same level of critical information regarding lens fit. The C measurement typically refers to the distance from the bottom of the lens to the lowest point of the frame, which can affect the positioning of progressive lenses, but again does not directly involve lens fit. Therefore, understanding and applying the effective diameter is key to achieving a successful and optimal fit for each individual lens/frame combination.