Patent ID: 7015940

Claim:
A color image forming apparatus for scanning a beam from at least one scanning optical apparatus on a plurality of image bearing members to thereby form a color image, said color image forming apparatus comprising: a light source; a deflecting element for deflecting and scanning a beam emitted from said light source; a scanning optical element for imaging said deflected beam into a spot shape on the scanned surfaces of said image bearing members; an optical element for synchronous detection for directing the deflected beam from said deflecting element to a sensor to take the timing of image writing beginning, wherein an optical axis of said optical element for synchronous detection is coincident with a principal ray of the deflected beam from said deflecting element; and a registration detecting unit for detecting a positional deviation in the main scanning direction of a marking of a predetermined shape formed on each of said image bearing members by each scanning optical apparatus is provided at a position corresponding to an image height separate from the optical axis of said scanning optical element; wherein said registration detecting unit is disposed so as to be capable of detecting two image heights substantially symmetrical with respect to the optical axis of said scanning optical element, and a correcting unit for effecting correction control of correcting the timing of image writing beginning by an amount corresponding to ΔA=K× (Δ1+Δ2)/2, where Δ1 is the amount of registration deviation at a first image height, Δ2 is the amount of registration deviation at a second image height, and K is a constant.