Image forming apparatus and image formation correcting method

An image formation correcting method of performing correction relative to image formation in an image forming apparatus which includes a photosensitive member extending in an axial direction;

• a conveying member configured to convey a recording medium in a predetermined conveyance direction, an exposure head disposed close to the photosensitive member and configured to form a latent image on the photosensitive member by performing exposure based on image data, a developing unit configured to develop the latent image formed on the photosensitive member, an image forming unit configured to form an image obtained by the development by the developing unit on the recording medium or the conveying member and a concentration sensor configured to detect an image and has a first detection width which is a detection width in the conveyance direction and a second detection width which is a detection width in the axial direction of the photosensitive member, the method comprising:
• controlling the exposure head and the image forming unit to form a first image for correction, which includes a straight line part extending in the axial direction of the photosensitive member and having a length equal to or larger than the second detection width of the concentration sensor, on the conveying member;
• correcting a developing bias to be applied to the developing unit for developing the latent image based on a detection result of a concentration of the first image for correction by the concentration sensor;
• and
• after correcting the developing bias, forming two or more types of second images for correction, each having a predetermined concentration, on the conveying member, and performing gamma correction based on detection results of the second images for correction by the concentration sensor;
• wherein the second image for correction is different from the first image for correction and includes a part which is not a straight line having a length equal to or larger than the second detection width of the concentration sensor.