1. An image forming apparatus that performs image formation on a recording sheet according to image data, comprising: a determination unit configured to, when performing the image formation, determine whether or not a processing instruction instructing execution of predetermined processing on the image data has been given; and a preview display control unit configured to, when it is determined by said determination unit that the predetermined processing instruction has been given, display an image to be formed on the recording sheet in the same size as when printed thereon, as a real-size preview image, before executing the image formation on the recording sheet according to image data on which the predetermined processing has been performed.