Optical scanner and image forming apparatus

An image forming apparatus comprising an optical scanner that optically scans a photosensitive medium, the optical scanner including

• a deflecting unit that deflects optical beams emitted from a light source at a constant angular velocity;
• and
• a scanning imaging lens that condenses the optical beams deflected on a surface to be scanned, the scanning imaging lens including at a first lens that is arranged closer to the deflection unit and a second lens that is arranged closer to the surface to be scanned, wherein
• the first lens is formed by resin molding, and has a positive refracting power in the horizontal scanning direction, with zero or close to zero refracting power in the vertical scanning direction, and has a function of correcting a constant velocity characteristic in the optical scanning,
• the second lens has a weak refracting power in the horizontal scanning direction and a strong positive refracting power in the vertical scanning direction, and
• when it is assumed that a deflection angle of optical beams deflected by the deflection unit is θ, an image height at the deflection angle θ of the light spot is H(θ), and an ideal image height at the deflection angle θ of the light spot is kθ, designating k as a constant,
• the linearity Lin of the scanning imaging lens defined by
• Lin=|[{(dH(θ)/dθ)/k}−1]|×100(%)
• satisfies a condition
• 2.0<Lin<10.0(%).