An example operation method of an image forming apparatus includes forming a test image with respect to a pattern used for diagnosing an image-quality state of a printout output by the image forming apparatus, obtaining a test signal corresponding to the test image via a sensor of the image forming apparatus, obtaining image-quality state information indicating an image-quality state of the test image based on the test signal corresponding, identifying a type of a screen corresponding to halftone information indicating a resolution of the printout based on the image-quality state information, and performing a first image forming operation according to the type of the screen.