Scanning optical system, optical scanner, and image forming apparatus having a biconvex lens having a change of curvature of the toric surface in the cross section in the sub-scanning direction along the main scanning direction is asymmetric with an optical axis of the lens

摘要

A scanning optical system includes converges, with a single lens, a divergent luminous flux, which is deflected in a main scanning direction by an optical deflector, on a surface of a scan target. The lens has two surfaces in a biconvex shape in both the main scanning direction and a sub-scanning direction, and at least one of the surfaces is a toric surface in which a line that connects, on a cross section in the main scanning direction, centers of curvature in a cross section in the sub-scanning direction is nonlinear, and change of the curvature of the toric surface in the cross section in the sub-scanning direction along the main scanning direction is asymmetric with an optical axis of the lens. The surfaces are anamorphic surfaces.