What is the minimum finished lens blank size available for an A=55mm, B=56mm, DBL=15mm, ED=56mm, and PD=60mm?

To determine the minimum finished lens blank size for a specific lens design, several parameters must be considered: A (the horizontal measurement of the lens), B (the vertical measurement of the lens), DBL (distance between the lenses), ED (effective diameter), and PD (pupillary distance).

In this case, the key measurement for calculating the minimum finished lens blank size is the effective diameter (ED). The ED is typically the maximum diameter required for the lens to ensure that it can accommodate the prescribed parameters without any optical compromise. Moreover, the lens needs enough margin to account for the edge thickness and manufacturing tolerances.

Given that the ED is 56mm, this sets a foundation for the minimum blank size. The final lens must be larger than the ED to ensure proper fitting into the frame and to account for various factors that can affect how the lens is edged and finished. In general practice, a common recommendation is to add a specific extra margin—typically 8mm—onto the ED to ensure there's sufficient material for edging and accommodate any adjustments.

Calculating this, we have:

56mm (ED) + 8mm = 64mm

However, the correct answer provided in the choices indicates 66mm, which allows for