An apparatus for forming an image is disclosed which includes: an input device inputting image data; an image processor processing the inputted image data; an image forming device forming the image on a recording medium, based on the processed image data; a lightness setting device setting a lightness value of the image to be formed on the recording medium by the image forming device; a first corrector correcting the inputted image data according to a first correction characteristic; a first characteristic modifier modifying the first correction characteristic, in response to setting of increasing the lightness value by the lightness setting device; a second corrector correcting the processed image data according to a second correction characteristic; and a second characteristic modifier modifying the second correction characteristic, in response to setting of decreasing the lightness value by the lightness setting device.