The number of X-ray photons is estimated that is required for the recording of object projections from which a sectional image of desired spatial and density resolution can be computed. We find that N_0,total  = const. n³ a² photons must enter the object slice, where n is the number of pixels along the image diameter, a is the number of gray-levels, and the constant is object specific. Of course, the X-ray dose additionally increases with decreasing beam width.