An Automatically-triggered digital video-imaging based code symbol reading system providing a high degree of adaptability to ambient illumination levels and having great dynamic range. The system includes an illumination subsystem, having an active illumination mode and an ambient illumination mode. During object illumination and image capturing operations, the exposure quality of a captured frames of digital video image data is analyzed, and based on the results of such exposure quality analysis, system control parameters (SCPs) are automatically reconfigured. In the event that the exposure quality analysis indicates excessive exposure levels in captured frames of digital video image data, then a new set of SCPs is reconfigured, controlling the illumination subsystem to operate in its ambient illumination mode, whereby, ambient illumination is allowed to flood the field of view and form digital images on the image detection array during object illumination and video-imaging operations.