| 摘要 |
PROBLEM TO BE SOLVED: To obtain a scanning optical system which is capable of making the spot diameter in a sub-scanning direction within an effective scanning region uniform, is compact and is optimum for high-fineness image recording and an image forming device using the same. SOLUTION: When a surface to be scanned is scanned by guiding the luminous flux emitted from a light source mean via first and second optical systems to a deflecting element and imaging the luminous flux reflected and deflected by the deflecting element onto the surface to be scanned by a third optical system, the third optical element has >=2 lenses and at least one lens La of the >=2 lenses is continuously changed in the radius of curvature in the sub- scanning direction of its lens face from on-axis toward the outside of the axis in the main scanning direction, and the change rate of the curvature in the sub-scanning direction within the one luminous flux on the lens face is so set as to attain <=10%. The shape in the main scanning direction of the lens La is so formed that the F number in the sub-scanning direction of the luminous flux incident on the surface to be scanned is approximately constant within the effective scanning region.
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