1. A method for determining noise in radiography comprising: acquiring at least two images, i−1 and i, of a same zone; coding the acquired images into digital images that can be identified with matrices having horizontal by vertical dimensions equal to N×M, each digital image being then formed by N.M dots, each dot of an image i being identifiable by its coordinates 0 <x<N, and 0 <y<M, this dot then being referred to as a dot Pi(x,y), each dot Pi(x,y) then having a corresponding value v which is the result of the acquisition of the image, the value v having a dynamic range from Vmin to Vmax; dividing the dynamic range Vmax-Vmin into sub-groups defined by a lower limit Bi and an upper limit Bs, the sub-groups having a null intersection, the joining of the sub-groups covering the dynamic range Vmax-Vmin, a dot of an image i then belonging to a given sub-group when Bi<&equals;Pi(x,y)<Bs, where Pi(x,y) is the gray level of the pixel of the image i with coordinates (x,y); computing, for at least one sub-group SG, of the mean standard deviation &sgr; of the values Pi(x,y)−Pi−1(x,y); discriminating the values Pi(x,y) of SG to keep only those values such that the criterion C: Pi(x,y)−Pi−1(x,y)<&mgr;(Pi(x,y)−Pi−1(x,y))&plus;k.&sgr;, is met and thus, a sub-group SG′ is obtained, where &mgr; is a mean value; applying the same processing operations to the sub-group SG′ as to the sub-group SG by iteration until a sub-group SG″, corresponding to an end-of-iteration criterion, is obtained; performing iterative processing operations on all the sub-groups defined in the dynamic range Vmax-Vmin and thus, for each sub-group, a mean standard deviation, associated with an x-axis value v&equals;(Bi&plus;Bs)/2, is obtained; and performing an operation of regression on the dots obtained at the previous step to determine the coefficients &agr;, &bgr; and &Ggr; of the noise function: &sgr;(v)&equals;&agr;.{square root}v&plus;&bgr;.v&plus;&Ggr; defining the noise for a given value v.