1. An image-capturing device that generates a wide dynamic range image through image synthesis, comprising: an image-capturing unit that obtains image data by executing photoelectric conversion on a light flux received at an image sensor; a dynamic range widening determination unit that determines that a dynamic range of an image is to be widened when a subject lightness value is equal to or lower than a lightness value upper limit which is greater than a predetermined lightness value provided that at least either the subject lightness value is equal to or greater than the predetermined lightness value or that an image-capturing frame rate is equal to or higher than a predetermined frame rate; an exposure control unit that controls exposure at the image sensor based upon determination results indicating whether or not to widen the dynamic range; an image synthesis unit that generates synthetic image data by combining image data in a plurality of frames captured over varying exposure times when a determination to widen the dynamic range is made; and a processing unit that displays or records the synthetic image data.