The smallest diameter blank for lens cutting is determined by what method?

The method for determining the smallest diameter blank for lens cutting involves doubling the decentration measurement and adding it to the Effective Diameter (E.D.) of the lens. This approach ensures that the final blank size accommodates both the necessary decentration due to the optical alignment of the lenses and the overall size of the lens required for proper fitting into the frames.

When lenses are prescribed, especially those requiring decentration (the horizontal or vertical shift of the optical center from the geometric center of the lens), it is crucial to account for this shift to avoid misalignment of the optical axes and to ensure optimal vision correction. The Effective Diameter takes into consideration the physical constraints of the lens and must be paired with the adjusted decentration to determine the minimum blank diameter.

Utilizing the decentration alone without accounting for the Effective Diameter would risk creating lenses that are too small or inadequately optimized for the frames accommodating them, potentially leading to visual problems or aesthetic concerns. Thus, this combination of doubling the decentration and adding it to the E.D. ensures a properly sized blank suitable for producing the final lenses.