1. An image processing device for generating combined image data using multi-viewpoint image data including image data acquired with different focal lengths, the image processing device comprising: a resolution converting unit configured to perform resolution conversion for at least part of image data in multi-viewpoint image data in accordance with a focal length to be output; and an image combining unit configured to generate combined image data with the focal length to be output using the resolution-converted image data, wherein at least part of image data is image data acquired with a focal length shorter than the focal length to be output, and wherein the resolution converting unit performs resolution conversion accompanied by enlargement for the image data acquired with the shorter focal length.