In a method for determination or adaptation of a shim for homogenization of a magnetic field of a magnetic resonance apparatus, which magnetic field is provided for the generation of magnetic resonance exposures of a specific examination region, an automatic determination, of a computer and/or supported by an operator on an image output unit of the computer, is made of a three-dimensional volume that is relevant for the determination or adaptation of the shim and that is matched to the examination region and/or an examination protocol by selection from an arbitrary morphology set of selectable volumes (which morphology set is not limited to specific shapes) and/or by generation of an arbitrarily three-dimensional volume not limited to specific shapes. The computer then automatically calculates the shim for the determined or selected three-dimensional volume.