1. A method for determining and evaluating a shim parameter set for controlling a shim device in a magnetic resonance apparatus, comprising the steps of: setting a shim device of a magnetic resonance apparatus using a first shim parameter set; measuring a first field distribution in a body region of a subject in the magnetic resonance apparatus encompassing a target volume from which magnetic resonance data are to be acquired; determining a second shim parameter set using an optimization algorithm that optimizes field homogeneity in the target volume based on said first shim parameter set and based on said first field distribution; determining a second field distribution based on said second shim parameter set; evaluating said first and second shim parameter sets by direct or indirect comparison of said first field distribution and said second field distribution in said target volume to obtain an evaluation result; and setting said shim device, to produce an optimally set shim device, based on said evaluation result.