1. A method of determining a magnification factor in a radiography apparatus comprising means for providing an X-ray source and means for acquiring images, the source and the image acquisition being mounted so as to rotate about at least one axis with respect to a support on which an object to be X-rayed is intended to be positioned, comprising: (a) acquiring at least two images corresponding to two different angular positions of the source and of the image acquisition with respect to the support; (b) identifying on these images projections of at least one point of the X-rayed object; an d (c) determining the magnification factor of at least one of the images, first, as a function of the angular displacement of the source and of the image acquisition between the acquisitions of the images in question and, secondly, as a function of the positions on these images of the identified projections.