1. A method to determine a strength of a magnetic interference field caused by an interference object located in a data acquisition volume in a magnetic resonance data acquisition unit, comprising the steps of: providing magnetic resonance image data to a processor, said magnetic resonance image data comprising image data acquired from a measurement volume, in which an interference object is located, of a magnetic resonance data acquisition unit, said interference object causing a magnetic interference field, and having a magnetic susceptibility that differs from a magnetic susceptibility of material surrounding the interference object; from said processor, making said image data available in a form allowing determination of at least one geometric variable of an image artifact in said image data caused by said magnetic interference field; from said image data in said form, determining said at least one geometric variable; and in a computerized computation unit, automatically determining the strength of the interference field caused by the interference object, based on the determined geometric variable of the image artifact.