Ophthalmic Optics Practice Exam

Minimum blank size is the stock lens blank diameter needed to produce the finished lens, factoring in how much the lens must be shifted within the blank (decentration) and a small finishing allowance. You add the final lens diameter, the total decentration required to align the lens with the frame, and a 2 mm margin for finishing. That 2 mm handles beveling, trimming, and small manufacturing variations. So the best expression is MBS = ED + total decentration + 2 mm. For example, if the final lens diameter (ED) is 60 mm and the lens must be decentered by 3 mm, the MBS would be 60 + 3 + 2 = 65 mm. Leaving out the decentration or the finishing margin could lead to a blank that’s too small to safely edge and fit.