1. A method for detecting objects used in a device for detecting objects, wherein the method comprises: acquiring an image having pixels; generating a plurality of image blocks, in which each of the image blocks corresponds to one of the pixels; filtering a specific image block of the image blocks by using N filtering parameters to generate N filtering results, in which the N filtering parameters gradually enhance the blurriness of the specific image block; computing N root mean square error (RMSE) values, in which the M-th RMSE value is computed according to the M-th and the (M−1)-th filtering results; computing a slope of an approximate line according to the RMSE values as the blurriness value of the specific image block; repeating the above steps to generate the blurriness values of all of the image blocks correspond to the pixels; and comparing each of the blurriness values with a threshold value to detect corresponding pixels which are parts of a sharp object and further detect an in-focus object.