Patent ID: 8705158

Claim:
A scanning optical device comprising: a light source; a first optical system configured to collect a light beam emitted from said light source, wherein said first optical system includes at least one movable optical element movable along an optical axis direction; a deflector configured to scanningly deflect the light beam from said first optical system; a second optical system configured to guide the light beam scanningly deflected by a deflecting surface of said deflector onto a surface to be scanned to form an image; a third optical system comprising a plurality lenses, including at least first two lenses aligned along a main-scan direction and at least second two lenses aligned along a sub-scan direction, configured to concurrently divide a same light beam scanningly deflected by the deflecting surface of the deflector and passed through an outside of an effective scan region of the second optical system and incident on the plurality of lenses, into a plurality of light beams with at least first two of the plurality of light beams arranged along the main-scan direction and at least second two of the plurality of light beams arranged along the sub-scan direction; a detecting device configured to detect the at least first two light beams arranged along the main-scan direction and the at least second two light beams arranged along the sub-scan direction, and detect positional information concerning a spacing in the main-scan direction between imaging positions of the at least first two light beams and a spacing in the sub-scan direction between the imaging positions of the at least second two light beams; an arithmetic unit configured to determine a focal shift direction and a focal shift amount in the main-scan direction, as well as a focal shift direction and a focal shift amount in the sub-scan direction, on the basis of the positional information detected by said detecting device; and a driving device configured to move, based on the determination made by the arithmetic unit, the movable optical element of said first optical system along the optical axis direction to correct the focal shift in the main-scan direction and the focal shift in the sub-scan direction.