1. A variable focus imaging lens assembly for an imaging system of an imaging-based bar code reader for imaging and decoding a target bar code, the imaging system generating image frames of a field of view of the imaging system and including an imaging sensor array and a decoder for decoding an image of the target bar code, the variable focus imaging lens assembly comprising: a lens assembly focusing light from the field of view onto the sensor array, the lens assembly including a lens movable along a path of travel; a drive mechanism driving the movable lens from a predetermined position along the path of travel; an automatic focusing system: energizing the drive mechanism and imaging system to move the movable lens from the predetermined position along its path of travel and generate a series of image frames; analyzing one or more image frames in a portion of the series of image frames to identify a region of interest by locating an image of a target bar code in the one or more analyzed image frames; estimating an amount of blurring in each of at least two subsequent image frames in a subsequent portion of the series of image frames within the identified region of interest of the one of more analyzed image frames, wherein the at least two of the subsequent image frames have two different focus planes, each focus plane corresponding to a different position along the path of travel of the movable lens in the lens assembly; selecting an image frame from the subsequent image frames of the series of image frames if the estimated amount of blurting in the identified region of interest in an image frame is less than a predetermined threshold; and utilizing the decoder to decode the image of the target bar code in the selected image frame.