Full-field kinematic measurements with one or several high-speed cameras is a promising non-invasive technique for the modal analysis of civil engineering structures. It raises however the question of the positioning of the cameras for an optimal geometric characterization of the mode shapes of interest. We address this question on a simplified model, from both experimental and numerical points of view. Standard perpective projection equations are first used. A mode-visibility criterion is proposed and illustrated for two modes and positions. Possible enrichments of the criterion are discussed.