A method of detecting generally rectangular objects in an image comprises determining candidate rectangles in the image based on detected corners in the image, ranking the candidate rectangles on the basis of a set of differentiating factors and detecting objects in the images based on the ranked candidate rectangles. A backing plate to which the permanent magnets are attached has channels within which adjusting elements are used to move the shim blocks with relation to the permanent magnets, preferably toward or away from the air gap. Any number of stages may be used. An apparatus of the present invention may include a processor and a machine-readable medium storing a set of instructions which cause the processor to unlock a locking device and activate a safety device. The wall section is elastically flexible and, in the mounted state, bent toward the end surface of the protuberance by the contractive force of the fastening element.