Fixation is the link between the observer and the events in the outside world and is a key element in the control strategy of an active vision system. The authors describe how the fixation process functions in human beings in general and discuss one of the essential components of fixation, vergence, in particular. They discuss the components of the human fixation model and through its geometry and organization describe a machine fixation model, running in real-time on a head-eye system. The authors are motivated by the need for gaze control in head-eye systems, which entails the need to clarify some fundamental relations in primate vergence movements, namely, the relations between disparity and blur and between vergence and accommodation. Following the geometry behind vergence movements, it is also suggested that the mechanism for disparity detection can use the symmetry present in the pattern of the left and right images through isolating vergence from version.>