Patent ID: 7460159

Claim:
An image forming apparatus comprising: a deflector that deflects optical beams modulated according to an image signal to perform scanning in a horizontal scanning direction to form an image on an image carrier; two optical beam detectors that respectively detect optical beams deflected by the deflector at two positions on a horizontal scanning line; a time difference measuring unit that measures the time difference between detection of the optical beams by the detectors; a magnification correcting unit that corrects the magnification of an image in the horizontal scanning direction based on the time difference measured by the time difference measuring unit, wherein the magnification correcting unit includes a frequency modulator that performs magnification correction of an image by changing a frequency of the image signal in units of a line or lines; and a phase modulator that performs magnification correction of the image by fixing the frequency and changing a cycle time of an optional pixel in units of pixel; a comparing unit that compares the phase adjustment amount calculated by the phase modulator with a preset specified value; and an arrangement that causes the phase modulator to perform the magnification correction of the image upon the comparing unit determining that the phase adjustment amount does not exceed the specified value, and, once the comparing unit determines that the phase adjustment amount exceeds the specified value, causes the frequency modulator to perform the magnification correction of the image.