Passive autofocusing is used in image acquisition devices (e.g., machine-readable symbol readers, cameras), and can be employed with shutter and/or event-based image sensors. An aimer pointer is easily detected in images, and one or more characteristics characterized at various focus positions of the optics. A size of a characteristic dimension and/or shape of the aimer pointer and/or a measure of sharpness of the aimer pointer is used to determine which image, and hence focus position, results in an optimized or even optimum (i.e., best) focus. The image acquisition system is then configured accordingly. Measuring aimer pointer size is typically less computationally intensive than conventional approaches. Use of a laser beam to produce the aimer pointer usually requires lower exposure time than other approaches. Image data can advantageously be windowed using a relatively small region of interest (ROI) based on a known aimer pointer position.