The invention relates to an optical system that is capable of providing a zoom lens having improved image-formation capability with a reduced number of lenses, and fabricating a slim yet high-performance digital or video camera. The optical system comprises a cemented lens having a cementing surface on its optical axis. Air contact surfaces on the light ray entrance and exit sides of the cemented lens G2 are aspheric, and at least one cementing surface in the cemented lens G2 satisfies condition (1). 6.4