Optical scanner and image forming apparatus

An optical scanner for scanning an image carrier comprising:

• an illuminant part emitting an optical beam;
• a deflecting part deflecting the optical beam;
• an image forming part focusing the deflected optical beam on the image carrier;
• and
• an optical axis adjustment part adjusting an optical axis of the optical beam with respect to a sub-scanning direction during an image writing operation of image data,
• the optical axis adjustment part comprising a prism for adjustment of a scanning position or a beam spot position of the optical beam on the image carrier with respect to a sub-scanning direction during the writing of image data by rotating the prism on an optical axis of the optical beam to deflect the incoming optical beam within a maximum deflection angle Φ, the maximum deflection angle Φ being defined in a formula Φ=(n−1)×α in which n is a refractive index of the prism and α is an apex angle of the prism, and the apex angle α of the prism satisfies:
• 0.1<β×tan[(n−1)×α]<1.0
• in which β is a lateral magnification of all optical systems with respect to the sub-scanning direction, to compensate for misalignment of the scanning position or beam spot position due to fluctuation in rotation of the image carrier.