Scanning optical system and image forming apparatus using the same

A scanning optical system comprising:

• a first optical system for shaping a light beam emitted from light source means and forming the light beam into a linear image, which is elongated in the same direction as a main scanning direction;
• a light deflector, which has a deflection surface at or near an imaging position of said first optical system, and which deflectively scans the incident light beam in the main scanning direction;
• a second optical system for forming the light beam deflected by said light deflector into an image on a scanned surface in the main scanning direction;
• and
• a third optical system for forming the light beam deflected by said light deflector into an image on the scanned surface in a subscanning direction and setting the deflection surface of said light deflector and the scanned surface optically so as to be conjugate with each other,
• wherein the light beam guided by said first optical system is made to strike said light deflector at a predetermined angle with respect to a plane perpendicular to a rotational axis of said light deflector,
• wherein said third optical system comprises at least one lens,
• wherein said at least one lens of said third optical system is positioned to make a light beam strike said at least one lens of said third optical system at a position shifted from a surface vertex in a subscanning cross-section of said at least one lens of said third optical system by a predetermined amount in the subscanning direction to correct a deterioration in a spot shape on a scanning end portion in the main scanning direction,
• wherein said second optical system is located closer to said light deflector than half the distance between said light deflector and the scanned surface, and
• wherein |φ2S|≦0.001 where φ2S is a refracting power of said second optical system in the subscanning direction.