Acquisition of a free viewpoint video (FVV) system is theoretically considered as a problem of plenoptic signal sampling. It is typically performed by using a regular camera grid. While a regular acquisition itself results in non-uniform sampling density, this non-uniformity does not match the scene complexity and frequency variations. This paper proposes an irregular acquisition method for optimum non-uniform plenoptic sampling corresponding to the variations of the scene complexity. Specifically, scene complexity is measured through analyzing DCT coefficients of reference images of the scene, describing the frequency behavior of the plenoptic signal over the scene space. An optimization model is formulated to calculate the optimum configurations of the acquisition cameras including positions and orientations. The theoretical analysis and numerical simulations demonstrate that the rendered video quality can be significantly improved (around 40% in mean PSNR) by employing the proposed irregular acquisition compared with the regular camera grid.