Method of calculating a finished lens

A system for producing a finished lens, with a predetermined shape, comprising one front face, one back face and a perimeter corresponding to said shape, the system comprising:

• a non-transitory computer readable medium including instructions that, when executed on a computational device, cause the computational device to perform steps of:
• providing a minimum edge thickness for the finished lens;
• determining a first curvature profile of a back surface to be machined on the back face of a lens blank, the lens blank having a convex front face and a concave back face;
• determining, free of frame shape data into which the lens is to be mounted, a prescription part on the lens that together with the front face fulfills prescription data of a wearer and a peripheral part on the lens by determining an isothickness curve corresponding to a value of thickness less than or equal to a predetermined thickness requirement, the prescription part being delimited by the isothickness curve and the peripheral part extending from the isothickness curve towards an outer edge of the lens;
• determining whether a thickness of the outer edge of a peripheral part of that lens blank machined on the back face with the first curvature profile at the peripheral part will be less than the minimum edge thickness if the back surface of the back face of the lens blank is manufactured using the first curvature profile at the peripheral part;
• and
• if the thickness of outer edge of the peripheral part will be less than the minimum edge thickness, determining a new curvature profile of the peripheral part of the back surface to be machined wherein the thickness in the peripheral part is greater or equal to the minimum edge thickness in order to avoid sharp edges.