1. A method for registering a two-dimensional image data set, generated using fan-shaped imaging rays, in the medical field, comprising: processing the two-dimensional image data set on the basis of a spatial transformation function which describes a spatial relative position between points which have been imaged using the fan-shaped imaging ray, and an imaging apparatus used for imaging, wherein processing the two-dimensional image data set includes ascertaining the spatial transformation function; and processing the two-dimensional image data set on the basis of an imaging transformation function which describes an imaging function of the imaging apparatus used for generating the two-dimensional image data set, which describes a spatial relationship between the actual spatial position of imaged points and an imaging location of the imaged points in a recording, wherein processing the two-dimensional image data set includes ascertaining the imaging transformation function, wherein geometric projections of points to be imaged, onto a fanning direction of the fanned ray, are taken into account for ascertaining the imaging function, and wherein ascertaining the imaging function comprises ascertaining a proportionality factor which describes a relationship between spatial positions of points to be imaged, relative to each other, at a certain height above a detection plane of the imaging apparatus, and detection positions of the imaged points, relative to each other, in a fanning direction of the fan-shaped imaging ray.