Nonlocal potentials can in general be transformed only approximately into local potentials with additional energy dependence. For the optical model Perey and Buck did an exact nonlocal calculation with the Schrödinger-equation in integro-differential form and also a good local approximation. Certain systematic deviations in the results (concerning the wave-functions) of both processes are of special interest in the shell model. Therefore the eigenvalue problem has been solved now with the most successful nonlocal potential, also used by Perey and Buck, both exactly and approximately, that is with the potential, which is nondiagonal, and its approximation, which is diagonal in local space. The solution of this approximate nonlocal potential serves again as zero order starting value ( E 0, u 0) for an iteration process, which now is twofold, in order to find that exact solution ( E, u). Thus we get together with the exact solution also the deviation of the approximate nonlocal (local-equivalent) one.