We propose to set in an easy formular every element of an X-Ray device : - X-Ray source, - X-Rayed body, - The receipt apparatus. We are then obliged to set a nomber of simplifying hypothesis : - The radiation energy is the same for every calculation, - The source is supposed to be pin-pointed and isotropic. To caracterize the details at the level of the X-Rayed object we introduce the notion of "object contrast". The reception system is defined by a gain conversion which expresses the probability of an X-photon unabsorbed by the body, proving at least one recorded event. Statistical considerations allow us to calculate the minimum surface of the detail to identify must have. This minimum, inverse resolution of the whole X-Ray apparatus, varies according to the number of photons provided by the source, the way the object modulates the beam, and the conversion gain of the reception system. For exemple, at a given energy - we can show that the resolution is maximum when this object has an optical thickness twice the mean pathlength of the radiation.