Real time response requires limiting the amount of information processed. In vision this means that you can not look everywhere at the same time. Applying this idea to stereo vision leads to a system in which reconstruction is limited to the region of a scene around a fixation point. Such fixation is achieved by controlling motors for vergence and focus.
Classic stereo correspondence reconstructs the scene in a reference frame based on the stereo cameras. Such an approach poses two major difficulties for active vision. Unfortunately, actively changing the vergence angle and focus modifies the camera parameters, making stereo reconstruction by classic techniques impossible. Active vision systems avoid reconstruction whenever possible. Indeed many visual control tasks such driving a car or grasping an object can be performed by servoing directly from measurements made in the image [Espiau et. al 92]. When 3D reconstruction is necessary, active vision systems exploit geometric invariant relations to reconstruct the scene using its own "intrinsic" coordinates [Crowley et al 93].
References:
[Crowley et al 93] J. L. Crowley, P. Bobet and C. Schmid , "Auto-Calibration of Cameras by Direct Observation of Objects", Image and Vision Computing, Vol 11, no. 2, March 1993.
[Espiau et. al] B. Espiau, F. Chaumette, and P. Rives. "A New Approach to Visual Servoing in Robotics", IEEE Trans. on Robotics and Automation, 8(3):313-326, June 1992.