A method and apparatus for determining a magnification factor in a radiography device of the type comprising an X-ray source and means for acquiring images placed facing the source, the source and the means for acquiring images 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. The method and the apparatus implementing the method comprises: acquiring at least two images corresponding to two different angular positions of the source and of the recording means with respect to the support; identifying on these images projections of at least one point of the X-rayed object; and 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 recording means between the acquisitions of the images in question and, secondly, as a function of the positions on these images of the identified projections.