numbers 11 12 13 denote respectively the front face the back face and the peripheral edge of the lens blank 1 referring to the position of the final eyeglass when used by a wearer the front face 11 is called thereafter top face and the rear face is called bottom face with respect to the implementation of the invention process thus the peripheral edge 13 connects the top face 11 to the bottom face 12 all around the lens blank 1 the peripheral edge 13 is commonly circular with diameter between 60 mm millimeter and 85 mm for example the lens blank material may be any one commonly used in optics the top face 11 comprises a convex surface portion 2 and a margin portion 3 which is located between the convex surface portion 2 and the peripheral edge 13 the convex surface portion 2 and the margin portion 3 form therebetween a re entrant angle along a depression track 4 within the context of the invention the angle u03b8 between the planes respectively tangential to the convex surface portion 2 and the margin portion 3 at the depression track 4 is less than 180 u00b0 when measured from outside the lens blank 1 the margin portion.