1. An image forming apparatus comprising: an image forming unit adapted to form a toner image, and control a contrast potential to be used for forming a toner image by the image forming unit according to input image data, the contrast potential being a difference voltage between (1) a surface potential of a photosensitive member charged by the image forming unit and exposed by a predetermined output of the image forming unit and (2) a developing bias to be applied for developing the photosensitive member with a toner by the image forming unit; a reading device adapted to read a toner image formed on a sheet by the image forming unit and detect a first density of the toner image formed on the sheet; a contrast potential determining unit adapted to determine a first contrast potential so that the first density of the toner image formed on the sheet becomes a predetermined density, based on a reading result of the reading device; a density detection unit adapted to detect a second density of a toner image formed by the image forming unit; a characteristic determining unit adapted to determine, based on second densities, detected by the density detecting unit, of a plurality of toner images formed using a plurality of different second contrast potentials by the image forming unit, a characteristic representing a relationship of (a) the plurality of different second contrast potentials and (b) the second densities of the plurality of toner images; and a correction unit adapted to correct the first contrast potential determined by the contrast potential determining unit, based on (i) the second density of a toner image formed by the image forming unit using the first contrast potential detected by the density detection unit and (ii) the characteristic determined by the characteristic determining unit.