1. An automatic testing system for surveillance camera installations, comprising: an input port to receive test or “probe” images from at least one camera in the installation; a computer memory to store a reference image from at least the one camera; a computer processor to compare a probe image with a reference image from the same camera and produce an output when maintenance is required for that camera; the comparison involving the steps of: extracting salient features from both the probe and reference images; calculating matching factors between the salient features extracted from both images; and, computing a decision about whether maintenance is required from the matching factors.