1. A method of determining a subtraction image from two successively formed images of an object, a sub-image being selected from each of the two images subdivided into pixels, each sub-image containing the same pixels and being compared with one another each time after having been shifted with respect to one another, for each sub-image there being determined a measure of the difference between pixel values of the pixels after each shift and also a shift vector, containing orthogonal direction components, for which the measure of the difference is smallest, after which the positions of pixels in an image are corrected by means of the locally determined shift vectors, for the determination of the measure of the difference the value of each pixel of one of the sub-images being increased or decreased by an amount if the sum of the position indices of the pixel is even or odd, respectively, after which a subtraction sub-image is determined and the number of sign changes between the adjacent pixels in the subtraction image is counted, and in a position where the counted number of sign changes exceeds a predetermined fraction of the maximum number of feasible sign changes the measure of the difference is deemed to be the smallest, characterized in that the numbers of sign changes associated with at least the last three positions of the one sub-image with respect to the other sub-image are stored, after which a reliability number is determined from the differences between the numbers, for each orthogonal component of the shift vector there being stored three numbers associated with the last three positions in each direction and a reliability number being formed for each direction component.