An imaging system includes a light source, an image sensor and a processing unit. The image sensor alternatively captures a first bright image, a first dark image, a second bright image and a second dark image, wherein the first bright image is captured with a first exposure time corresponding to activation of the light source within a first time interval, the first dark image is captured with the first exposure time corresponding to deactivation of the light source within the first time interval, the second bright image is captured with a second exposure time corresponding to activation of the light source within a second time interval, and the second dark image is captured with the second exposure time corresponding to deactivation of the light source within the second time interval, wherein the second exposure time is longer than the first exposure time. The processing unit adjusts the second exposure time according to an object image size in the second dark image, and controls the image sensor to stop capturing the first bright and dark images with the first exposure time when no object image is contained in the second dark image.