A method of fitting a holding block to a semifinished ophthalmic lens blank intended to have a predetermined prism, which method includes the following steps: positioning the blank on a fixed base so that the finished face of the blank bears conjointly on a plurality of bearing points of the base, defining an orientation of the holding block, orienting the holding block in the defined manner, and fixing the holding block to the finished face, the step of defining the orientation of the holding block including the following steps: taking account of the three-dimensional shape of the finished face and the position of the bearing points, deducing therefrom the orientation of the finished face, taking account of a predetermined prism, and deducing from the orientation of the finished face and the predetermined prism the orientation of the holding block.