An optical scanner that may assemble the lens design for varying the focus and resolution, having a light source, a reflection compound mirror, a charge coupled device, a basic objective lens and a compound lens. The basic objective lens is designed by simulation software. According to the lens design theory, the compound lens is designed. By incorporating the basic objective lens and the compound lens, different resolutions such as 1200 dpi, 1600 dpi and 2400 dpi of the optical scanner are obtained without redesigning the lens device, the current specification of the optical scanner is also varied.