If ED = 50 mm and decentration per lens d = 1 mm, what is the minimum blank size?

• A. 54 mm
• B. 52 mm
• C. 50 mm
• D. 56 mm

Answer: (A)

When you choose a lens blank, you need enough material to allow the lens center to be shifted during edging without risking edge contact. Decentration per lens means the optical center can move by d in both directions, and you effectively need margin on both sides of the blank. That adds up to 4d extra over the ED. So the minimum blank size is ED + 4d. With ED = 50 mm and d = 1 mm, that yields 50 + 4(1) = 54 mm.