Conventional radiographs (digitized or not) do not provide information about the depths of details and structures because they are two-dimensional projections of three-dimensional bodies. Taking advantage of the finite size of the X-ray source and the divergent nature of the X-ray beam, a radiograph can be processed by two-dimensional digital filtering techniques, so that the image of a particular layer is improved, while the others are degraded. This technique is referred to as a Tomographic Filtration Process (TFP). This paper explains the mathematical and physical foundations of the method and the engineering considerations in the design and realization of tomographic filters. Theoretical comparisons between conventional radiography, standard tomography and tomographic filtering are discussed in terms of the thickness of the tomographic layer, the rate of change of the Modulation Transfer Function (MTF) and the signal to noise ratios. Finally, experimental results are shown to demonstrate the effect of tomographic filtering at different depths.