1. An image processing method comprising: an image acquiring step of acquiring (a) a captured image produced by an image pickup system including an optical system and an image sensor, (b) an image pickup condition for the captured image, and (c) information for specifying a size of a pixel of the image sensor; a coefficient acquiring step of acquiring coefficient data corresponding to the image pickup condition; a filter producing step of producing an image restoration filter based on the coefficient data and a frequency characteristic of an alias generated by the image sensor, where the frequency characteristic of the alias is identified according to the information for specifying the size of the pixel of the image sensor; and a restoring step of performing an image restoration process on the captured image by using the image restoration filter to produce a restored image.