IMAGE FORMING APPARATUS AND IMAGE FORMING APPARATUS CONTROL METHOD

PROBLEM TO BE SOLVED: To perform magnification correction in the main scanning direction without using a face detection sensor of a polygon mirror.SOLUTION: An image forming apparatus comprises: a rotary polygon mirror for scanning a light beam in the main scanning direction; a light detection part for detecting the light beam on a start end side and on a terminating end side of the main scanning direction so as to include the main scanning range of an image carrier; a measurement part for measuring a time difference (scanning time in the main scanning direction) between the light beam detection on the start end side and the light beam detection on the terminating end side of the light detection part; a storage part for storing magnification correction data previously created to correct the magnification in the main scanning direction so that the scanning length becomes constant on each reflection face of the rotary polygon mirror; a reflection face determination part for determining a correspondence between the magnification correction data and each reflection face of the rotary polygon mirror according to the difference in the scanning time in the main scanning direction among each reflection face of the rotary polygon mirror measured by the measurement part and the difference in the scanning time in the main scanning direction among each reflection face of the rotary polygon mirror previously measured; and a correction part for correcting the magnification in the main scanning direction by adjusting an exposure dot position interval of image data for each reflection face of the rotary polygon mirror with the determined magnification correction data for each reflection face.