Patent ID: 7382390

Claim:
An image forming apparatus for forming an image by scanning on a photosensitive member in a main-scanning direction with a plurality of beams aligned in a sub-scanning direction, comprising: a pattern forming unit adapted to form, with beams at both ends in the sub-scanning direction, patterns on the photosensitive member which are used for detecting a difference of scanning length in the main-scanning direction between the beams; a position detection unit adapted to detect positions of a start point and an end point in the main-scanning direction of the patterns on the photosensitive member formed with the beams at both ends; a difference of scanning length calculating unit adapted to calculate a difference of scanning length in the main-scanning direction between the beams at both ends based on the detected positions of the start and end positions on the photosensitive member; a correction amount calculating unit adapted to calculate correction amounts for respectively correcting scanning lengths of the plurality of beams based on the calculated difference of scanning length between the beams at both ends; and a correcting unit adapted to correct scanning lengths of the plurality of beams respectively based on the calculated correction amounts, wherein, when a difference of scanning length in the main-scanning direction between the beams at both ends is taken as ΔL and the number of beams is taken as N, said correction amount calculating unit calculates a correction amount Li of a scanning length in a main-scanning direction of a beam located in the i-th position in the sub-scanning direction by Li=ΔL×i/(N−1).