The invention discloses an active auto-focus tracking system using a time-of-flight (ToF) depth sensor. The ToF sensor serves as a rangefinder and also provides environmental depth information. The active auto-focus system includes a compressive tracking algorithm to track the movement of an object. The active auto-focus tracking system can acquire subject depth information regardless of various lighting conditions, subject positions, and visual patterns. Moreover, the system can perform an improved acquisition of focusing distance in both normal and low-level lighting conditions.