| 摘要 |
An object of the present invention is to provide a projection optical system having a large numerical aperture in which the maximum effective diameter of lens of the optical system is satisfactorily small. There is provided a projection optical system for projecting an image on a first surface onto a second surface, comprising a first lens group G1 of positive refracting power including two or more positive lenses, a second lens group G2 of negative refracting power including two or more negative lenses, a third lens group G3 of positive refracting power including three or more positive lenses, a fourth lens group G4 of negative refracting power including two or more negative lenses, and a fifth lens group G5 of positive refracting power including at least six or more consecutive positive lenses, in the named order from the first surface side to the second surface side, wherein either the fourth lens group G4 or the fifth lens group G5 has one aspherical surface *, the fifth lens group G5 has an aperture stop AS inside thereof, a portion of a diverged light flux right before the aperture stop AS has a first air lens LA of negative refracting power, the radius of curvature rA1 of the lens surface on the first surface side of the first air lens LA is positive, and a portion of a converged light flux at the rear of the aperture stop AS has a second air lens LB of negative refracting power.
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