The present disclosure provides a detection system, which includes an image sensor, a lens device, and a processor. The image sensor is configured to take a first picture of a foreground object and a background object. The lens device is attached to the image sensor and configured to allow the foreground object to form a clear image on the first picture and the background object to form a blurred image on the first picture. The processor is configured to determine the image of the foreground object by analyzing the sharpness of the images of the first pictures.