Overt visual attention is the act of directing eyes towards a given area. These eye movements are characterized by saccades and fixations. A debate currently concerns the role of visual fixations. Do they all have the same role in free viewing of natural scenes? Recent studies suggest that there exist at least two kinds of visual fixations called focal and ambient ones. The former would be used to inspect accurately local areas whereas the latter would be used to grab the context of the scene. We investigate in this paper an automatic solution to cluster visual fixations in two groups using four types of natural scene images. We give new evidences supporting a focal-ambient dichotomy. Our clustering reveals that the determining factor is the amplitude of saccade. The dependence on the low-level visual features and the time course of these two kinds of visual fixations are examined. Results demonstrate that there is an interplay between both fixation populations and that focal fixations would be much more dependent on low-level visual features and centred than ambient ones.