1. An image capturing device comprising: an acquisition unit that acquires a plurality of image data representing mutually different images by performing a plurality of image captures of an object at different positions in a direction parallel to an optical axis of an optical system; an extraction unit that extracts a specified object image included in each of the images represented by the plurality of image data acquired by the acquisition unit; and a synthesis unit that produces blurred background image data representing a synthesized blurred background image by: (i) changing sizes of the images represented by the plurality of image data acquired by the acquisition unit so that the specified object images extracted by the extraction unit are superimposed on one another, and (ii) calculating luminance values of pixels of the synthesized blurred background image by calculating respective mean values of luminance values of corresponding pixels of image data representing the images whose sizes were changed.