Besides its historic relevance, this text introduces to the theory of tomographic imaging. It starts with the question about the required number of X-ray photons for certain spatial and grey-level resolutions of the reconstructed sectional image. The systems theoretical analysis of the now outdated “coded source” and “coded aperture” schemes provides general insights to the rôle projection angles play for the depth resolution and it nicely relates to the problem of depth resolution in microscopy. A survey of analog approaches to tomographic imaging reveals the most common principles of image reconstruction from projections. Finally, “spectral tomosynthesis” by coherent-optical computing is proposed for image reconstruction.