Optical scanning apparatus and image forming apparatus

An image forming apparatus configured to form an image on a recording sheet, comprising:

• a photosensitive member configured to form an electrostatic latent image thereon;
• a developing device configured to supply a developer to the electrostatic latent image to form a developed image on the photosensitive member;
• a transfer device configured to transfer the developed image onto the recording sheet;
• and
• an optical scanning apparatus, comprising:
• a semiconductor laser;
• a coupling lens made of resin, the coupling lens having a diffractive optical element on at least one side thereof, the coupling lens being configured to convert light emitted from the semiconductor laser into a beam;
• a polygon mirror configured to deflect the beam into a main scanning direction;
• a motor configured to rotate the polygon mirror;
• an optical scanning system configured to focus the beam deflected by the polygon mirror on the photosensitive member, the optical scanning system including a first scanning lens made of resin;
• a housing retaining the semiconductor laser, the coupling lens, the polygon mirror, the motor, and the optical scanning system, the housing including a base on one side thereof in an axial direction, the axial direction being a direction of a rotation axis of the polygon mirror;
• a cover covering the other side of the housing in the axial direction;
• and
• a partition enclosing a space where the polygon mirror and the motor are located between the base and the cover, the partition having an opening, through which the deflected beam passes,
• wherein the first scanning lens closes the opening, the first scanning lens having an optical surface located closest to the polygon mirror among a plurality of optical surfaces included in the optical scanning system, and
• wherein shifting amounts of a focus position of the beam with respect to a reference imaging plane in the main scanning direction are in relations ΔA<0 and ΔA<ΔB<ΔC, where
• ΔA mm is a shifting amount in a case where the semiconductor laser, the coupling lens, and the optical scanning system are at a normal ambient temperature,
• ΔB mm is a shifting amount in a case where the semiconductor laser, the coupling lens, and the optical scanning system are at an upper-limit ambient temperature, and
• ΔC mm is a shifting amount in a case where the semiconductor laser and the coupling lens are at the normal ambient temperature and the first scanning lens is at a first temperature, the first temperature being higher than the normal ambient temperature.