We intend to build a vision system that will allow dynamic 3D-perception of objects of interest. More specifically, we discuss the idea of using 3D visual cues when tracking a visual target, in order to recover some of its 3D characteristics (depth, size, kinematic information). The basic requirements for such a 3D vision module to be embedded on a robotic head are discussed. The experimentation reported here corresponds to an implementation of these general ideas, considering a calibrated robotic head. We analyse how to make use of such a system for (1) detecting 3D-objects of interest, (2) recovering the average depth and size of the tracked objects, (3) fixating and tracking such objects, to facilitate their observation.