Patent ID: 7391431

Claim:
An image forming apparatus comprising: a deflector configured to deflect optical beams modulated according to an image signal to thereby scan a surface of an image carrier in a horizontal scanning direction to form an image on the image carrier; an optical beam detector arranged on either side of the image carrier along the horizontal scanning direction, wherein the optical beam detectors are configured to detect optical beams deflected by the deflector; a time difference measuring unit configured to measure a time difference between detections of the optical beams by optical beam detectors; and a magnification correcting unit configured to correct, based on the time difference, a magnification in the horizontal scanning direction of the image on the image carrier, wherein the magnification correcting unit includes a main position correcting unit configured to perform magnification correction of the image by changing a beam spot position interval on a scanning line in units of a line or lines; a sub position correcting unit configured to perform magnification correction of the image by changing a beam spot position interval on a scanning line in units of pixel; and a position adjustment amount-determining unit that determines whether a beam spot position adjustment amount corresponding to each of a plurality of developing colors preset by the sub position correcting unit exceeds a threshold preset for each of the plurality of developing colors, wherein the magnification correction of the image by the sub position correcting unit is changed over to the magnification correction of the image by the main position correcting unit based on a determination result of the position adjustment amount-determining unit.