1. An image processing device comprising: a parallax control processing section configured to perform an image processing by modifying a previously measured parallax magnitude with a parallax magnitude adjustment factor, wherein the parallax magnitude adjustment factor is based on at least one of: a measured amount of change in a focal length of an image-pickup lens from a first point in time to a later second point in time; and a measured amount of change in a subject distance from the first point in time to the second point in time, wherein the parallax magnitude adjustment factor is set to offset a change in parallax magnitude resulting from the change in either the focal length of the image-pickup lens or the change in the subject distance over time.