An automatic exposure system for an imaging-based bar code reader. The automatic identification system includes: an aiming apparatus generating a beam to aid in aiming the system at a target object when the system is actuated; an imaging system including a pixel array, a focusing lens to focus an image of the target object onto the pixel array; and an automatic exposure system to determine an integration time for capturing an image of the target object. The automatic exposure system determines an integration time by: projecting an aiming pattern on the target object and capturing an image of the aiming pattern; determining a target distance from the imaging system to the target object based on a location of the aiming pattern within the captured image; determining a gain-integration time product utilizing an equation wherein the gain-integration time product is a function of a predetermined target image brightness and the target distance; and determining the integration time by selecting a gain value and solving for integration time given the gain-integration time product.