1. An image-pickup apparatus which obtains a synthesized image whose exposure has been corrected by synthesizing a plurality of images taken successively, comprising: a focusing unit which operates a focusing operation to move a focus lens system of the image-pickup apparatus based on a signal of a focused region; an obtainment unit which obtains a focused condition within the focused region; a detection unit which detects, with respect to a reference image, amounts of displacement on other images, wherein the detection unit divides the reference image into a first region including the focused region by the focusing operation of the lens system on the reference image and a second region other than the first region, and detects the amounts of displacement based on a specific part in the second region of the reference image and specific parts of the other images in respective regions corresponding to the second region; and a synthesis unit which synthesizes the plurality of images based on the results of detection of the detection unit