Auto-Focusing in Extreme Zoom Surveillance: A System Approach with Application to Faces Yi Yao, Besma Abidi, Michael Tousek, Mongi Abidi The University of Tennessee, Knoxville, TN, 37996 Abstract. Auto-focusing is an indispensable function for imaging systems used in surveillance and object tracking. In this paper, we conduct a study of an im- age-based passive auto-focusing control for high magnification (>50×) systems using off-the-shelf telescopes and digital camcorders with applications to long range near-ground surveillance and face tracking. Considering both speed of convergence and robustness to image degradations induced by high system magnifications and long observation distances, we introduce an auto-focusing mechanism suitable for such applications, including hardware design and algo- rithm development. We focus on the derivation of the transition criteria follow- ing maximum likelihood (ML) estimation for the selection of adaptive step sizes and the use of sharpness measures for the proper evaluation of high mag- nification images. The efficiency of the proposed system is demonstrated in real-time auto-focusing and tracking of faces from distances of 50m~300m. 1 Introduction PTZ cameras are commonly used in indoor and outdoor surveillance systems [1, 2]. Most of these cameras provide a maximum optical magnification in the range of 20×~25×. However, for long range surveillance and target identification, this magni- fication is insufficient. To extend the optical zoom capability beyond 50×, we ex- ploited digital imaging systems with scopes (telescopes and spotting scopes) in near- ground surveillance [3]. As the first step in building a high magnification imaging system using off-the-shelf equipment, we studied several setups based on various scopes, eyepieces, and digital cameras/camcorders [3]. The chosen combination of a Celestron telescope (GPS 11) and a Sony camcorder (TRV730) is able to achieve a system magnification of up to 1800×, which is sufficient for observing human faces from a distance of 1km. For this system to be useful in real-time tracking scenarios, it is critical to keep the moving target in focus. In a composite imaging system, the focus of the scope plays a dominant role. Although digital cameras are equipped with auto-focusing function, scopes are available only with manual focus control. To facilitate complete remote and automatic control of this high magnification imaging system, the auto-focusing capability needs to be integrated.