Adaptive optics can be used to improve the quality of optical imaging instruments otherwise limited in resolution by atmospheric turbulence. To achieve a given level of resolution improvement at the lowest cost, design of adaptive mirrors must be carefully optimized. We have calculated the mean value of the optical transfer function (OTF) of an instrument corrected by adaptive optics using the OTF as a criterion for resolution optimization and describing perturbations through their phase structure function. A fourparameter numerical program has been written to compute the OTF. Assuming that the optics diameter and the perturbation Fried's parameter are known, it then becomes possible to optimize the system by a proper choice of the number of actuators and of the influence diameter, a parameter which characterizes the size of the required corrections.