1. An image capturing apparatus comprising: an image pickup configured to capture an image of a subject and output image data; an image processor configured to perform image processing on image data output from the image pickup; a combiner configured to combine a plurality of pieces of image data; a corrector configured to correct the combined exposure image data; and a controller configured to perform control so that the image pickup performs image capturing with an image pickup element of the image pickup being exposed to light, thereby obtaining a plurality of pieces of exposure image data, the image processor performs image processing on the plurality of pieces of exposure image data, the combiner combines the plurality of pieces of exposure image data output from the image processor so as to generate combined exposure image data, the image pickup performs image capturing without the image pickup element being exposed to light, thereby obtaining a plurality of pieces of dark image data, the combiner generates one piece of dark reference image data by using the plurality of pieces of dark image data, the image processor performs, on the one piece of dark reference image data, image processing that uses a parameter based on a parameter of the image processing performed on each of the plurality of pieces of exposure image data, and outputs a plurality of pieces of processed dark image data corresponding to the plurality of pieces of exposure image data, the combiner combines the plurality of pieces of processed dark image data output from the image processor so as to generate combined dark image data, and the corrector corrects the combined exposure image data in accordance with the combined dark image data.